Juniper this week said it signed an agreement to acquire NetScreen Technologies for $4 billion in stock.NetScreen is a developer of network security and access products for enterprises and carriers. NetScreen's security appliances and systems are designed to provide network- and application-level protection.The company, with revenue of $223 million and 900 employees, is considered one of the top four or five IPSec VPN vendors. In October, NetScreen\u00a0bought start-up Neoteris\u00a0and its Secure Sockets Layer remote access technology for $265 million.NetScreen targets the high end of the enterprise market and recently reworked its products to appeal to midsized companies. Seventy-five percent of its revenue comes from enterprises while sales to service providers make up the remaining 25%.Most of Juniper\u2019s revenue comes from service providers, but the company has also been engaged with enterprises in the government, defense, financial and educational research verticals - most recently the GIG-BE win with the Defense Information Systems Agency. The NetScreen deal, Juniper\u2019s largest acquisition, is resounding affirmation that Juniper plans to focus more intently on this market.Indeed, the deal represents Juniper\u2019s about-face from its pledge not to compete with its service provider customers by targeting the enterprise market like its rival Cisco. Juniper CEO Scott Kriens attempted to explain this reversal and its likely reception from service provider customers. During a conference call on the NetScreen deal, Kriens offered a rambling response\u00a0about how the distinction between the enterprise and carrier markets was \u201cblurring\u201d or \u201coutdated.\u201d\u201cThe distinction is not campus vs. carriers,\u201d Kriens claimed. \u201cThat\u2019s the old way of looking at the problem. It\u2019s not physical\u00a0- it\u2019s virtual. It\u2019s better defined by the requirement for mission-critical networks (where) the problems are important, complex and rapidly changing.\u201dThe problems also add up to a $10 billion addressable market for the Juniper\/NetScreen combination where the demand from customers is \u201curgent\u201d and \u201cimmediate,\u201d Kriens says. Juniper will address these requirements with direct and indirect sales forces from both companies.NetScreen will form the Security Products Group within Juniper. NetScreen President and CEO Robert Thomas will head up the group and report to Kriens. Juniper has been addressing the security needs of its customers with enhancements to its routers.Juniper stock will be exchanged for NetScreen stock at a fixed exchange ratio of 1.404 shares of Juniper common stock for each outstanding share of NetScreen common stock. The acquisition is expected to close in the second quarter and is subject to customary closing conditions, including approval by the stockholders of both companies and regulatory approvals.Senior Editor Tim Greene contributed to this story.