
<PAGE>

                                                              NO. CSW-9

THE SECURITIES REPRESENTED BY THIS CERTIFICATE HAVE BEEN ACQUIRED FOR INVESTMENT
AND HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933 ("SECURITIES ACT")
OR ANY STATE SECURITIES LAWS. SUCH SECURITIES MAY NOT BE SOLD OR TRANSFERRED IN
THE ABSENCE OF SUCH REGISTRATION OR AN EXEMPTION THEREFROM UNDER THE SECURITIES
ACT AND ANY APPLICABLE STATE SECURITIES LAWS.

                        WARRANT TO PURCHASE 12,500 SHARES
                               OF COMMON STOCK OF
                                  F5 LABS, INC.
       (VOID AFTER THE EARLIER OF 2-25-2009 OR INITIAL REGISTRATION UNDER
                              THE SECURITIES ACT.)

         This certifies that PSINet Inc., a New York corporation (the "Holder")
having a place of business at 510 Huntmar Park Drive, Herndon, VA, 20170, or its
assigns, for value received, is entitled to purchase from F5 Labs, Inc., a
Washington corporation (the "Company"), having a place of business at 200 First
Avenue West, Suite 500, Seattle, WA 98119, a maximum of twelve thousand five
hundred (12,500) fully paid and nonassessable shares of the Company's Common
Stock ("Common Stock") for cash at a price of $8.00 per share (the "Stock
Purchase Price") at the earlier of (i) the time the Company files a registration
statement covering the Common Stock under the Securities Act, or (ii) at any
time or from time to time after December 31, 1999 up to and including 5:00 p.m.
(Pacific time) November 1, 2008. This Warrant will expire on the earlier of such
registration or November 1, 2008 ("Expiration Date"). In the event that the
Expiration Date of this Warrant falls on a day which is not a Business Day, the
Expiration Date shall be adjusted to the Business Day immediately following such
Expiration Date. As used herein, the term "Business Day" means each day other
than a Saturday, Sunday or other day on which banks in the location of the
principal offices of the Company are legally authorized to close. This Warrant
may be exercised upon surrender to the Company at its principal office (or at
such other location as the Company may advise the Holder in writing) of this
Warrant properly endorsed with the Form of Subscription attached hereto duly
filled in and signed and, if applicable, upon payment in cash or by check of the
aggregate Stock Purchase Price for the number of shares for which this Warrant
is being exercised determined in accordance with the provisions hereof. The
Stock Purchase Price and the number of shares purchasable hereunder are subject
to adjustment as provided in Section 3 of this Warrant.

         This Warrant is subject to the following terms and conditions:

         1. EXERCISE; ISSUANCE OF CERTIFICATES; PAYMENT FOR SHARES.

         1.1 GENERAL. This Warrant is exercisable at the option of the holder of
record hereof, as described above, up to the Expiration Date for all or any part
of the shares of Common Stock (but not for a fraction of a share) which may be
purchased hereunder. The Company agrees that the shares of Common Stock
purchased under this Warrant shall be and are deemed to be issued to the Holder
hereof as the record owner of such shares as of the close of business on the
date on which this Warrant shall have been surrendered, properly endorsed, the
completed, 



<PAGE>


executed Subscription Form delivered and payment made for such shares.
Certificates for the shares of Common Stock so purchased, together with any
other securities or property to which the Holder hereof is entitled upon such
exercise, shall be delivered to the Holder hereof by the Company at the
Company's expense within a reasonable time after the rights represented by this
Warrant have been so exercised. In case of a purchase of less than all the
shares which may be purchased under this Warrant, the Company shall cancel this
Warrant and execute and deliver a new Warrant or Warrants of like tenor for the
balance of the shares purchasable under the Warrant surrendered upon such
purchase to the Holder hereof within a reasonable time. Each stock certificate
so delivered shall be in such denominations of Common Stock as may be requested
by the Holder hereof and shall be registered in the name of such Holder.

         1.2 NET ISSUE EXERCISE. Notwithstanding any provisions herein to the
contrary, if the fair market value of one share of the Company's Common Stock is
greater than the Stock Purchase Price (at the date of calculation as set forth
below), in lieu of exercising this Warrant for cash, the Holder may elect to
receive shares equal to the value (as determined below) of this Warrant (or the
portion thereof being canceled) by surrender of this Warrant at the principal
office of the Company together with the properly endorsed Form of Subscription
and notice of such election in which event the Company shall issue to the Holder
a number of shares of Common Stock computed using the following formula: 

               X = Y(A-B)
                   ------
                      A


        Where     X=       the number of shares of Common Stock to be issued
                           to the Holder

                  Y=       the number of shares of Common Stock purchasable
                           under the Warrant or, if only a portion of the
                           Warrant is being exercised, the portion of the
                           Warrant being canceled (at the date of such
                           calculation)

                  A=       the fair market value of one share of the Company's
                           Common Stock (at the date of such calculation)

                  B=       Stock Purchase Price (as adjusted to the date of
                           such calculation)

         2. For purposes of the above calculation, the fair market value of one
share of Common Stock shall be determined by the Company's Board of Directors in
good faith; provided, however, that in the event the Common Stock is traded on a
securities exchange or the Nasdaq National Market System, the value shall be
deemed to be the average of the closing sale prices for the Common Stock over
the 5-day period ending one (1) days prior to the proposed effective date for
the net issue exercise described above. SHARES TO BE FULLY PAID; RESERVATION OF
SHARES. The Company covenants and agrees that all shares of Common Stock which
may be issued upon the exercise of the rights represented by this Warrant will,
upon issuance, be duly authorized, validly issued, fully paid and nonassessable
and free from all preemptive rights of 


                                       2
<PAGE>


any stockholder and free of all taxes, liens and charges with respect to the
issue thereof. The Company further covenants and agrees that, during the period
within which the rights represented by this Warrant may be exercised, the
Company will at all times have authorized and reserved, for the purpose of issue
or transfer upon exercise of the subscription rights evidenced by this Warrant,
a sufficient number of shares of authorized but unissued Common Stock, or other
securities and property, when and as required to provide for the exercise of the
rights represented by this Warrant. The Company will take all such action as may
be necessary to assure that such shares of Common Stock may be issued as
provided herein without violation of any applicable law or regulation, or of any
requirements of any domestic securities exchange upon which the Common Stock may
be listed; provided, however, that the Company shall not be required to effect a
registration under Federal or State securities laws with respect to such
exercise. The Company will not take any action which would result in any
adjustment of the Stock Purchase Price (as set forth in Section 3 hereof) if the
total number of shares of Common Stock issuable after such action upon exercise
of all outstanding warrants, together with all shares of Common Stock then
outstanding and all shares of Common Stock then issuable upon exercise of all
options and upon the conversion of all convertible securities then outstanding,
would exceed the total number of shares of Common Stock then authorized by the
Company's Articles of Incorporation, as amended. The Company agrees that its
issuance of this Warrant shall constitute full authority to its officers who are
charged with the duty of executing stock certificates to execute and issue the
necessary certificates for shares of Common Stock upon the exercise of this
Warrant.

         3. ADJUSTMENT OF STOCK PURCHASE PRICE AND NUMBER OF SHARES. The Stock
Purchase Price and the number of shares purchasable upon the exercise of this
Warrant shall be subject to adjustment from time to time upon the occurrence of
certain events described in this Section 3. Upon each adjustment of the Stock
Purchase Price, the Holder of this Warrant shall thereafter be entitled to
purchase, at the Stock Purchase Price resulting from such adjustment, the number
of shares obtained by multiplying the Stock Purchase Price in effect immediately
prior to such adjustment by the number of shares purchasable pursuant hereto
immediately prior to such adjustment, and dividing the product thereof by the
Stock Purchase Price resulting from such adjustment. 

         3.1 SUBDIVISION OR COMBINATION OF STOCK. In case the Company shall at
any time subdivide its outstanding shares of Common Stock into a greater number
of shares, the Stock Purchase Price in effect immediately prior to such
subdivision shall be proportionately reduced, and conversely, in case the
outstanding shares of Common Stock of the Company shall be combined into a
smaller number of shares, the Stock Purchase Price in effect immediately prior
to such combination shall be proportionately increased.

         3.2 DIVIDENDS IN COMMON STOCK, OTHER STOCK, PROPERTY, RECLASSIFICATION.
If at any time or from time to time the Holders of Common Stock (or any shares
of stock or other securities at the time receivable upon the exercise of this
Warrant) shall have received or become entitled to receive, without payment
therefor: 


                                       3
<PAGE>


         (A) Common Stock or any shares of stock or other securities which are
at any time directly or indirectly convertible into or exchangeable for Common
Stock, or any rights or options to subscribe for, purchase or otherwise acquire
any of the foregoing by way of dividend or other distribution;

         (B) any cash paid or payable otherwise than as a cash dividend; or 

         (C) Common Stock or additional stock or other securities or property
(including cash) by way of spin-off, split-up, reclassification, combination of
shares or similar corporate rearrangement (other than shares of Common Stock
issued as a stock split or adjustments in respect of which shall be covered by
the terms of Section 3.1 above), then and in each such case, the Holder hereof
shall, upon the exercise of this Warrant, be entitled to receive, in addition to
the number of shares of Common Stock receivable thereupon, and without payment
of any additional consideration therefor, the amount of stock and other
securities and property (including cash in the cases referred to in clause (b)
above and this clause (c)) which the Holder would hold on the date of such
exercise had the Holder been the holder of record of such Common Stock as of the
date on which holders of Common Stock received or became entitled to receive
such shares or all other additional stock and other securities and property. 

         3.3 REORGANIZATION, RECLASSIFICATION, CONSOLIDATION, MERGER OR SALE. If
any recapitalization, reclassification or reorganization of the capital stock of
the Company, or any consolidation or merger of the Company with another
corporation, or the sale of all or substantially all of its assets or other
transaction shall be effected in such a way that holders of Common Stock shall
be entitled to receive stock, securities, or other assets or property (a
"Change"), then lawful and adequate provisions shall be made by the Company
whereby the Holder hereof shall, until the consummation of such Change, provided
adequate notice thereof is given as provided below, have the right to elect to
purchase and receive (in lieu of the shares of the Common Stock of the Company
immediately theretofore purchasable and receivable upon the exercise of the
rights represented hereby) such shares of stock, securities or other assets or
property as may be issued or payable with respect to or in exchange for a number
of outstanding shares of such Common Stock equal to the number of shares of such
stock immediately theretofore purchasable and receivable upon the exercise of
the rights represented hereby. Unless exercised prior to such Change, provided
adequate notice thereof is given as provided below, this Warrant will expire.

         3.4 NEW ISSUANCE OR SALE. If the Company shall issue or sell any shares
of Common Stock, excluding any stock issued under any employee stock option or
other compensation plan, for a consideration per share less than the Stock
Purchase Price in effect at such date, as specified herein, or without
condiseration, such Stock Purchase Price shall forthwith be reduced to a price
determined by dividing:

         (A) an amount equal to (i) the number of shares of Common Stock
outstanding immediatley prior to such issuance or sale multiplied by the Stock
Purchase Price, plus, (ii) the consideration, if any, received by the Company
upon such issuance or sale, by



                                       4
<PAGE>


         (B) the number of shares of Common Stock outstanding immediately after
such issuance or sale.

         Upon each such subsequent issue and sale of shares of Common Stock of
the Company for a consideration per share less than the Stock Purchase Price
then in effect as so adjusted, or without consideration, the Stock Purchase
Price as so adjusted shall be forthwith reduced in the same manner as specified
above.

         3.5 OTHER EVENTS. If any event occurs as to which, in the opinion of
the Board of Directors of the Company, the other provisions of this Section 3
are not strictly applicable or if strictly applicable, would not adequately
protect from dilution the exercise rights of the Holder in accordance with the
intent and principles of such provision, then the Board of Directors of the
Company shall make an equitable adjustment in the application of such
provisions, so as to protect the exercise rights as aforesaid, but in no event
shall such adjustment have the effect off overriding the Stock Purchase Price.

NOTICES OF ADJUSTMENT.

         (A) Immediately upon any adjustment in the number or class of shares
subject to this Warrant and of the Stock Purchase Price, the Company shall give
written notice thereof to the Holder, setting forth in reasonable detail and
certifying the calculation of such adjustment.

         (B) The Company shall give written notice to the Holder at least 10
business days prior to the date on which the Company closes its books or takes a
record for determining rights to receive any dividends or distributions. (C) The
Company shall also give written notice to the Holder at least 30 business days
prior to the date on which a Change shall take place. 4. ISSUE TAX. The issuance
of certificates for shares of Common Stock upon the exercise of the Warrant
shall be made without charge to the Holder of the Warrant for any issue tax
(other than any applicable income taxes) in respect thereof; provided, however,
that the Company shall not be required to pay any tax which may be payable in
respect of any transfer involved in the issuance and delivery of any certificate
in a name other than that of the then Holder of the Warrant being exercised.

         5. CLOSING OF BOOKS. The Company will at no time close its transfer
books against the transfer of any warrant or of any shares of Common Stock
issued or issuable upon the exercise of any warrant in any manner which
interferes with the timely exercise of this Warrant. 

         6. NO VOTING OR DIVIDEND RIGHTS; LIMITATION OF LIABILITY. Subject to
Aticle 3 of this Warrant, nothing contained in this Warrant shall be construed
as conferring upon the Holder hereof the right to vote or to consent or to
receive notice as a stockholder of the Company or any other matters or any
rights whatsoever as a stockholder of the Company. No dividends or interest
shall be payable or accrued in respect of this Warrant or the interest


                                       5
<PAGE>


represented hereby or the shares purchasable hereunder until, and only to the
extent that, this Warrant shall have been exercised. No provisions hereof, in
the absence of affirmative action by the holder to purchase shares of Common
Stock, and no mere enumeration herein of the rights or privileges of the holder
hereof, shall give rise to any liability of such Holder for the Stock Purchase
Price or as a stockholder of the Company, whether such liability is asserted by
the Company or by its creditors. 

         7. WARRANTS TRANSFERABLE. Subject to compliance with applicable federal
and state securities laws, this Warrant and all rights hereunder are
transferable, in whole or in part, without charge to the holder hereof (except
for transfer taxes), only with the prior written consent of the Company,
provided that no such consent is required after the Company has filed a
registration statement under the Securities Act and such registration statement
has become effective. Subject to the foregoing, each taker and holder of this
Warrant, by taking or holding the same, consents and agrees that this Warrant,
when endorsed in blank, shall be deemed negotiable, and that the holder hereof,
when this Warrant shall have been so endorsed, may be treated by the Company, at
the Company's option, and all other persons dealing with this Warrant, as the
absolute owner hereof for any purpose and as the person entitled to exercise the
rights represented by this Warrant. 

         8. RIGHTS AND OBLIGATIONS SURVIVE EXERCISE OF WARRANT. The rights and
obligations of the Company, of the holder of this Warrant and of the holder of
shares of Common Stock issued upon exercise of this Warrant shall survive the
exercise of this Warrant. 

         9. MODIFICATION AND WAIVER. This Warrant and any provision hereof may
be changed, waived, discharged or terminated only by an instrument in writing
signed by the party against which enforcement of the same is sought. 

         10. NOTICES. Any notice, request or other document required or
permitted to be given or delivered to the holder hereof or the Company shall be
delivered personally, by confirmed facsimile or shall be sent by certified mail,
postage prepaid, to each such holder at its address as shown on the books of the
Company or to the Company at the address indicated therefor in the first
paragraph of this Warrant or such other address as either may from time to time
provide to the other. 

         11. BINDING EFFECT ON SUCCESSORS. All of the obligations of the Company
relating to the Common Stock issuable upon the exercise of this Warrant shall
survive the exercise and termination of this Warrant. All of the covenants and
agreements of the Company shall inure to the benefit of the successors and
assigns of the holder hereof. 

         12. DESCRIPTIVE HEADINGS AND GOVERNING LAW. The description headings of
the several sections and paragraphs of this Warrant are inserted for convenience
only and do not constitute a part of this Warrant. This Warrant shall be
construed and enforced in accordance with, and the rights of the parties shall
be governed by, the laws of the State of Washington, without regard to conflicts
of laws principles. 


                                       6
<PAGE>


         13. LOST WARRANTS. The Company represents and warrants to the Holder
hereof that upon receipt of evidence reasonably satisfactory to the Company of
the loss, theft, destruction, or mutilation of this Warrant and, in the case of
any such loss, theft or destruction, upon receipt of an indemnity reasonably
satisfactory to the Company, or in the case of any such mutilation upon
surrender and cancellation of such Warrant, the Company, at its expense, will
make and deliver a new Warrant, of like tenor and amount, in lieu of the lost,
stolen, destroyed or mutilated Warrant. 

         14. FRACTIONAL SHARES. No fractional shares shall be issued upon
exercise of this Warrant. The Company shall, in lieu of issuing any fractional
share, pay the holder entitled to such fraction a sum in cash equal to such
fraction multiplied by the then effective Stock Purchase Price. 

         15. REPRESENTATIONS AND WARRANTIES OF THE HOLDER.

         15.1 PURCHASE FOR OWN ACCOUNT. The Holder represents that it is
acquiring this Warrant and the Common Stock issuable upon conversion of the
Warrant (collectively, the "Securities") solely for its own account and
beneficial interest for investment and not for sale or with a view to
distribution of the Securities or any part thereof, has no present intention of
selling (in connection with a distribution or otherwise), granting any
participation in, or otherwise distributing the same, and does not presently
have reason to anticipate a change in such intention. The foregoing shall in no
way predjudice the Holder's rights at all times to sell or other wise dispose of
all or any part of such securities under a registration under the Securities Act
or an exemption from such registration.

         15.2 INFORMATION AND SOPHISTICATION. The Holder acknowledges that it
has received all the information it has requested from the Company and considers
necessary or appropriate for deciding whether to acquire the Warrant. The Holder
represents that it has had an opportunity to ask questions and receive answers
from the Company regarding the terms and conditions of the offering of the
Warrant and to obtain any additional information necessary to verify the
accuracy of the information given the Holder. The Holder further represents that
it has such knowledge and experience in financial and business matters that it
is capable of evaluating the merits and risk of this investment. 

         15.3 ABILITY TO BEAR ECONOMIC RISK. The Holder acknowledges that
investment in the Warrant involves a high degree of risk, and represents that it
is able, without materially impairing its financial condition, to hold the
Securities for an indefinite period of time and to suffer a complete loss of its
investment. 

         15.4 FURTHER LIMITATIONS ON DISPOSITION. The Holder has been informed
that under the Securities Act, the Securities must be held indefinitely unless
they are subsequently registered under the Securities Act or unless an exemption
from such registration (such as Rule 144) is available with respect to any
proposed transfer or disposition by the Holder of the Securities. The Holder
further agrees that the Company may refuse to permit the Holder to sell,
transfer or dispose of the Securities (except as permitted under Rule 144)
unless there is in effect 



                                       7
<PAGE>

a registration statement under the Securities Act and any applicable state
securities laws covering such transfer, or unless the Holder furnishes an
opinion of counsel reasonably satisfactory to counsel for the Company, to the
effect that such registration is not required. 

         15.5 LIMITATION ON DISPOSITION POST REGISTRATION. The Holder
acknowledges and agrees that the Company (or a representative of the
underwriters) may, in connection with the Company's registration of any
securities of the Company under the Securities Act, require that the Holder not
sell, dispose of, transfer, make any short sale of, grant any option for the
purchase of, or enter into any hedging or similar transaction with the same
economic effect as a sale, any shares of Common Stock or other securities of the
Company held by the Holder, for a period of time specified by the underwriter(s)
(not to exceed one hundred eighty (180) days) following the effective date of
the registration statement of the Company filed under the Securities Act. The
Holder further agrees to execute and deliver such other agreements as may be
reasonably requested by the Company and/or the underwriter(s) which are
consistent with the foregoing or which are necessary to give further effect
thereto. In order to enforce the foregoing covenant, the Company may impose
stop-transfer instructions with respect to the Holder's Common Stock until the
end of such period.

         15.6 EXPERIENCE. The Holder is an "accredited investor" as such term is
defined in Rule 501 under the Securities Act.

         IN WITNESS WHEREOF, the Company has caused this Warrant to be duly
executed by its officers, thereunto duly authorized this 25th day of February ,
1999.

                                                  F5 LABS, INC.


                                                  By: /s/ Jeffrey Hussey
                                                      Jeffrey Hussey

                                                  Title: CEO

                                                  ACKNOWLEDGED AND AGREED:

                                                  PSINET, INC.

                                                  By:    /s/ Michael Mael
                                                     --------------------------
                                                  Title: V.P. Web Services
                                                        -----------------------


                                       8
<PAGE>

                                    EXHIBIT A

                                SUBSCRIPTION FORM

                                                  Date:                  , 
                                                        -----------------  -----


F5 Labs, Inc.
200 First Avenue West
Suite 500
Seattle, WA 98119
Attn:  Brian Dixon

Ladies and Gentlemen:

/ /      The undersigned hereby elects to exercise the warrant issued to it by
         F5 Labs, Inc. (the "Company") and dated November 1, 1998, Warrant No.
         CS-___ (the "Warrant") and to purchase thereunder
         __________________________________ shares of the Common Stock of the
         Company (the "Shares") at a purchase price of _________ Dollars ($____)
         per Share (the "Purchase Price").

/ /      The undersigned hereby elects to convert _______________________
         percent (____%) of the value of the Warrant pursuant to the provisions
         of Section 1.2 of the Warrant.

         Please issue a new Warrant for the unexercised portion of the attached
         Warrant in the name of the undersigned or in such other name as is
         specified below.
         -----------------------------------
                                    (Name)

Pursuant to the terms of the Warrant, the undersigned has delivered the Purchase
Price herewith in full in cash or by certified check or wire transfer. The
undersigned also makes the representations set forth on the attached Exhibit B
of the Warrant.

                                              Very truly yours,

                                              PSINet Inc.

                                              By:
                                                 -----------------------------
                                              Title:
                                                    --------------------------


                                       1
<PAGE>


                                    EXHIBIT B

                            INVESTMENT REPRESENTATION

THIS AGREEMENT MUST BE COMPLETED, SIGNED AND RETURNED TO F5 LABS, INC. ALONG
WITH THE SUBSCRIPTION FORM BEFORE THE COMMON STOCK ISSUABLE UPON EXERCISE OF THE
WARRANT DATED NOVEMBER 1, 1998, WILL BE ISSUED.

                                                                          , 19
                                                                  ---- ---    --
F5 Labs, Inc.
200 First Avenue West
Suite 500
Seattle, WA 98119
Attn:  Brian Dixon

Ladies and Gentlemen:

         The undersigned, PSINet Inc.or its assigns (the "Purchaser"), intends
to acquire up to ___________ (___________) shares of the Common Stock (the
"Common Stock") of F5 Labs, Inc. (the "Company") from the Company pursuant to
the exercise or conversion of certain Warrants to purchase Common Stock held by
Purchaser. The Common Stock will be issued to Purchaser in a transaction not
involving a public offering and pursuant to an exemption from registration under
the Securities Act of 1933, as amended (the "1933 Act") and applicable state
securities laws. In connection with such purchase and in order to comply with
the exemptions from registration relied upon by the Company, Purchaser
represents, warrants and agrees as follows:

         Purchaser is acquiring the Common Stock for its own account, to hold
for investment, and Purchaser shall not make any sale, transfer or other
disposition of the Common Stock in violation of the 1933 Act or the General
Rules and Regulations promulgated thereunder by the Securities and Exchange
Commission (the "SEC") or in violation of any applicable state securities law.
The foregoing shall in no way prejudice the Holder's rights at all times to sell
or otherwise dispose of all or any part of such securities under a registration
under the Securities Act or an exemption from such registration.

         Purchaser has been advised that the Common Stock has not been
registered under the 1933 Act or state securities laws on the ground that this
transaction is exempt from registration, and that reliance by the Company on
such exemptions is predicated in part on Purchaser's representations set forth
in this letter.

         Purchaser has been informed that under the 1933 Act, the Common Stock
must be held indefinitely unless it is subsequently registered under the 1933
Act or unless an exemption from such registration (such as Rule 144) is
available with respect to any proposed transfer or disposition by Purchaser of
the Common Stock. Purchaser further agrees that the Company may refuse to permit
Purchaser to sell, transfer or dispose of the Common Stock (except as permitted
under Rule 


                                       1
<PAGE>


144) unless there is in effect a registration statement under the
1933 Act and any applicable state securities laws covering such transfer, or
unless Purchaser furnishes an opinion of counsel reasonably satisfactory to
counsel for the Company, to the effect that such registration is not required.

         Purchaser also understands and agrees that there will be placed on the
certificate(s) for the Common Stock, or any substitutions therefor, a legend
stating in substance:

                  "The shares represented by this certificate have not been
         registered under the Securities Act of 1933, as amended (the
         "Securities Act"), or any state securities laws. These shares have been
         acquired for investment purposes only and may not be sold or otherwise
         transferred in the absence of an effective registration statement for
         these shares under the Securities Act and applicable state securities
         laws, or an opinion of counsel satisfactory to the Company that
         registration is not required and that an applicable exemption is
         available."

         Purchaser has carefully read this letter and has discussed its
requirements and other applicable limitations upon Purchaser's resale of the
Common Stock with Purchaser's counsel.

                                      Very truly yours,

                                      PSINet Inc.

                                      By:
                                         ------------------------------

                                      Title:
                                            ---------------------------




                                       2

<PAGE>

                    Amendment to PSINet Inc. Warrant

This Amendment to PSINet Inc. Warrant ("Amendment") by and between F5 Networks,
Inc., (formerly F5 Labs, Inc.) a Washington corporation, ("F5")  and PSINet
Inc., a New York corporation, ("PSI") is effective as of March 1, 1999. This
Amendment amends the terms of that certain Warrant dated February 25, 1999 to
purchase 12,500 shares of F5 common stock that was granted by F5 to PSI
("Warrant").

F5 and PSI agree that the term "Expiration Date" as used in the Warrant will be 
amended to mean the earlier of the date that is two years from the effective 
date of F5's initial registration under the Securities Act or February 25, 
2009. All other terms and conditions of the Warrant will remain in full force 
and effect.

Acknowledged and Agreed:


F5 Networks, Inc. 


By:    /s/  Robert Chamberlain
    ------------------------------
     Robert Chamberlain
     Vice President of Finance and 
     Chief Financial Officer


PSINet Inc.


By: /s/ Kathleen B. Horne
   -------------------------------
   Kathleen B. Horne
   Vice President and
   Deputy General Counsel

