Exhibit 99.1

FOR IMMEDIATE RELEASE

CONTACT:		Investor Relations
		John Eldridge
		(206) 272-6571
		j.eldridge@f5.com
		Public Relations
		Alane Moran
		(206) 272-6850
		a.moran@f5.com

F5 Networks Announces Results for Second Quarter of Fiscal 2011

SEATTLE, WA—April 20, 2011— For the second quarter of fiscal 2011, F5 Networks, Inc. (NASDAQ: FFIV) announced revenue of $277.6 million, up 3.2 percent from $268.9 million in the prior quarter and 34.7 percent from $206.1 million in the second quarter of fiscal 2010.

GAAP net income was $55.6 million ($0.68 per diluted share), compared to $55.7 million ($0.68 per diluted share) in the prior quarter and $33.1 million ($0.41 per diluted share) in the second quarter a year ago.

Excluding the impact of stock-based compensation net of tax, non-GAAP net income was $71.5 million ($0.88 per diluted share), compared to $72.2 million ($0.88 per diluted share) in the prior quarter and $45.2 million ($0.56 per diluted share) in the second quarter of fiscal 2010.

“F5’s solid sequential and year-over-year revenue growth in the second quarter was driven by continuing strength in the Americas and APAC,” said John McAdam, president and chief executive officer. Revenue from the Americas grew 4 percent from the prior quarter and 38 percent from the second quarter of fiscal 2010. APAC revenue increased 10 percent from the prior quarter and 68 percent year-over-year. Japan revenue was down 6 percent sequentially and flat with revenue in the second quarter of last year. In EMEA, revenue grew 22 percent year-over-year but was essentially flat with the prior quarter.

“Overall revenue growth and stable gross margins enabled us to add 125 employees in the second quarter and maintain our non-GAAP operating margin at just under 38 percent,” McAdam said.

During the quarter F5 generated $91 million in cash from operations, and after repurchasing 404,933 shares of our outstanding common stock the company ended the quarter with $997 million in cash and investments.

For the current quarter, ending June 30, management has set a revenue goal of $287 million to $292 million with a GAAP earnings target of $0.69 to $0.71 per diluted share. Excluding stock-based compensation expense, the company’s non-GAAP earnings target is $0.89 to $0.91 per diluted share.

A reconciliation of the company’s expected GAAP and non-GAAP earnings is provided in the following table:

		Three months ended
		June 30, 2011
Reconciliation of Expected Non-GAAP Third Quarter Earnings		Low				High
Net income		$	56.4			$	58.2
Stock-based compensation expense, net of tax		$	16.4			$	16.4
Non-GAAP net income excluding stock-based compensation expense		$	72.8			$	74.6
Net income per share — diluted		$	0.69			$	0.71
Non-GAAP net income per share — diluted		$	0.89			$	0.91

About F5 Networks

F5 Networks is the global leader in Application Delivery Networking (ADN), focused on ensuring the secure, reliable, and fast delivery of applications. F5’s flexible architectural framework enables community-driven innovation that helps organizations enhance IT agility and dynamically deliver services that generate true business value. F5’s vision of unified application and data delivery offers customers an unprecedented level of choice in how they deploy ADN solutions. It redefines the management of application, server, storage, and network resources, streamlining application delivery and reducing costs. Global enterprise organizations, service and cloud providers, and Web 2.0 content providers trust F5 to keep their business moving forward. For more information, go to www.f5.com.

You can also follow @f5networks on Twitter or visit us on Facebook for more information about F5, its partners, and technology. For a complete listing of F5 community sites, please visit www.f5.com/news-press-events/web-media/community.html.

Forward Looking Statements

Statements in this press release concerning the continuing strength of F5’s business, sequential growth, the target revenue and earnings range, share amount and share price assumptions, demand for application delivery networking and storage virtualization products and other statements that are not historical facts are forward-looking statements. Such forward-looking statements involve risks and uncertainties, as well as assumptions and other factors that, if they do not fully materialize or prove correct, could cause the actual results,

performance or achievements of the company, or industry results, to be materially different from any future results, performance or achievements expressed or implied by such forward-looking statements. Such factors include, but are not limited to: customer acceptance of our new traffic management, security, application delivery, WAN optimization and storage virtualization offerings; the timely development, introduction and acceptance of additional new products and features by F5 or its competitors; competitive pricing pressures; increased sales discounts; uncertain global economic conditions which may result in reduced customer demand for our products and services and changes in customer payment patterns; F5’s ability to sustain, develop and effectively utilize distribution relationships; F5’s ability to attract, train and retain qualified product development, marketing, sales, professional services and customer support personnel; F5’s ability to expand in international markets; the unpredictability of F5’s sales cycle; the share repurchase program; future prices of F5’s common stock; and other risks and uncertainties described more fully in our documents filed with or furnished to the Securities and Exchange Commission. All forward-looking statements in this press release are based on information available as of the date hereof and qualified in their entirety by this cautionary statement. F5 assumes no obligation to revise or update these forward-looking statements.

GAAP to non-GAAP Reconciliation

F5’s management evaluates and makes operating decisions using various operating measures. These measures are generally based on the revenues of its products, services operations and certain costs of those operations, such as cost of revenues, research and development, sales and marketing and general and administrative expenses. One such measure is net income excluding stock-based compensation, which is a non-GAAP financial measure under Section 101 of Regulation G under the Securities Exchange Act of 1934, as amended. This measure consists of GAAP net income excluding, as applicable, stock-based compensation. Net income excluding stock-based compensation (non-GAAP) is adjusted by the amount of additional taxes or tax benefit that the company would accrue if it used non-GAAP results instead of GAAP results to calculate the company’s tax liability. Stock-based compensation is a non-cash expense that F5 has accounted for since July 1, 2005 in accordance with the fair value recognition provisions of Financial Accounting Standards Board (“FASB”) Accounting Standards Codification (“ASC”) Topic 718 Compensation — Stock Compensation (“FASB ASC Topic 718”).

Management believes that net income excluding stock-based compensation (non-GAAP) provides useful supplemental information to management and investors regarding the performance of the company’s business operations and facilitates comparisons to the company’s historical operating results. Although F5’s management finds this non-GAAP measure to be useful in evaluating the performance of the business, management’s reliance

on this measure is limited because items excluded from such measures could have a material effect on F5’s earnings and earnings per share calculated in accordance with GAAP. Therefore, F5’s management will use its non-GAAP earnings and earnings per share measures, in conjunction with GAAP earnings and earnings per share measures, to address these limitations when evaluating the performance of the company’s business. Investors should consider these non-GAAP measures in addition to, and not as a substitute for, financial performance measures in accordance with GAAP.

F5 believes that presenting its non-GAAP measure of earnings and earnings per share provides investors with an additional tool for evaluating the performance of the company’s business and which management uses in its own evaluation of the company’s performance. Investors are encouraged to look at GAAP results as the best measure of financial performance. For example, stock-based compensation is an obligation of the company that should be considered and each line item is important to financial performance generally. However, while the GAAP results are more complete, the company provides investors this supplemental measure since, with reconciliation to GAAP, it may provide additional insight into its operational performance and financial results.

# # #

F5 Networks, Inc.
Condensed Consolidated Balance Sheets
(unaudited, in thousands)

		March 31,				September 30,
		2011				2010
Assets
Current assets
Cash and cash equivalents		$	206,215			$	168,754
Short-term investments			269,041				259,742
Accounts receivable, net of allowances of $2,652 and $4,319			142,903				112,132
Inventories			18,154				18,815
Deferred tax assets			8,759				8,767
Other current assets			45,767				37,745
Total current assets			690,839				605,955
Property and equipment, net			37,533				34,157
Long-term investments			521,920				433,570
Deferred tax assets			39,608				37,864
Goodwill			234,700				234,700
Other assets, net			13,923				15,946
Total assets		$	1,538,523			$	1,362,192
Liabilities and Shareholders’ Equity
Current liabilities
Accounts payable		$	30,827			$	21,180
Accrued liabilities			63,101				61,768
Deferred revenue			248,157				204,137
Total current liabilities			342,085				287,085
Other long-term liabilities			18,952				16,153
Deferred revenue, long-term			65,183				55,256
Total long-term liabilities			84,135				71,409
Commitments and contingencies
Shareholders’ equity
Preferred stock, no par value; 10,000 shares authorized, no shares outstanding			—				—
Common stock, no par value; 200,000 shares authorized 80,709 and 80,355 shares issued and outstanding			515,973				517,215
Accumulated other comprehensive loss			(4,632	)			(3,241	)
Retained earnings			600,962				489,724
Total shareholders’ equity			1,112,303				1,003,698
Total liabilities and shareholders’ equity		$	1,538,523			$	1,362,192

F5 Networks, Inc.
Condensed Consolidated Statements of Operations
(unaudited, in thousands, except per share amounts)

		Three Months Ended								Six Months Ended
		March 31,								March 31,
		2011				2010				2011				2010
Net revenues
Products		$	173,710			$	129,559			$	345,202			$	248,777
Services			103,862				76,509				201,304				148,447
Total			277,572				206,068				546,506				397,224
Cost of net revenues (1)
Products			31,423				27,419				63,037				53,461
Services			19,250				13,997				36,599				27,084
Total			50,673				41,416				99,636				80,545
Gross Profit			226,899				164,652				446,870				316,679
Operating expenses (1)
Sales and marketing			89,332				69,644				176,157				135,286
Research and development			34,507				29,134				67,113				55,854
General and administrative			19,846				16,016				40,530				31,969
Total			143,685				114,794				283,800				223,109
Income from operations			83,214				49,858				163,070				93,570
Other income, net			1,568				2,291				4,113				3,996
Income before income taxes			84,782				52,149				167,183				97,566
Provision for income taxes (1)			29,207				19,005				55,945				35,143
Net Income		$	55,575			$	33,144			$	111,238			$	62,423
Net income per share — basic		$	0.69			$	0.42			$	1.38			$	0.79
Weighted average shares — basic			80,809				79,394				80,726				79,147
Net income per share — diluted		$	0.68			$	0.41			$	1.36			$	0.77
Weighted average shares — diluted			81,622				80,737				81,670				80,630
Non-GAAP Financial Measures
Net income as reported		$	55,575			$	33,144			$	111,238			$	62,423
Stock-based compensation expense, net of tax (2)			15,912				12,038				32,448				24,168
Net income excluding stock-based compensation (non-GAAP)		$	71,487			$	45,182			$	143,686			$	86,591
Net income per share excluding stock-based compensation (non-GAAP) — diluted		$	0.88			$	0.56			$	1.76			$	1.07
Weighted average shares — diluted			81,622				80,737				81,670				80,630
(1)   Includes stock-based compensation as follows:
Cost of net revenues		$	2,167			$	1,685			$	4,395			$	3,284
Sales and marketing			8,359				6,377				17,092				13,094
Research and development			5,589				4,579				11,477				9,448
General and administrative			5,651				3,880				11,742				7,759
Tax effect of stock-based compensation			(5,854	)			(4,483	)			(12,258	)			(9,417	)
		$	15,912			$	12,038			$	32,448			$	24,168

F5 Networks, Inc.
Condensed Consolidated Statements of Cash Flows
(unaudited, in thousands)

		Six Months Ended
		March 31,
		2011				2010
Operating activities
Net income		$	111,238			$	62,423
Adjustments to reconcile net income to net cash provided by operating activities:
Realized gain on disposition of assets and investments			(182	)			(13	)
Stock-based compensation			44,706				33,585
Provisions for doubtful accounts and sales returns			(14	)			1,257
Depreciation and amortization			10,536				12,088
Deferred income taxes			(1,080	)			5,340
Loss on auction rate securities put option			—				19
Gain on trading auction rate securities			—				(19	)
Changes in operating assets and liabilities, net of amounts acquired:
Accounts receivable			(30,757	)			16,331
Inventories			661				(2,653	)
Other current assets			(7,798	)			(4,481	)
Other assets			(140	)			(2,038	)
Accounts payable and accrued liabilities			13,253				(13,283	)
Deferred revenue			53,945				43,356
Net cash provided by operating activities			194,368				151,912
Investing activities
Purchases of investments			(441,160	)			(331,410	)
Sales and maturities of investments			342,207				230,595
Investment of restricted cash			38				(22	)
Acquisition of intangible assets			(80	)			—
Purchases of property and equipment			(11,704	)			(6,840	)
Net cash used in investing activities			(110,699	)			(107,677	)
Financing activities
Excess tax benefits from stock-based compensation			16,286				9,700
Proceeds from the exercise of stock options and purchases of stock under employee stock purchase plan			9,218				19,149
Repurchase of common stock			(71,526	)			(35,000	)
Net cash used in financing activities			(46,022	)			(6,151	)
Net increase in cash and cash equivalents			37,647				38,084
Effect of exchange rate changes on cash and cash equivalents			(186	)			(397	)
Cash and cash equivalents, beginning of period			168,754				110,837
Cash and cash equivalents, end of period		$	206,215			$	148,524