(FORTUNE Magazine) – How do you go from zero to nearly $5 billion in a day? How about by beating Cisco at its own game?

At least that's what seems to have worked for newly public Juniper Networks. The three-year-old Mountain View, Calif., company makes cutting-edge routers (which direct traffic on computer networks)--a field long dominated by Cisco. And way back in September it beat Cisco to market with a high-speed router system called the M40 geared specifically for the Internet. Cisco's version is still on the drawing board. "They've one-upped Cisco in that sense," says Steven Tuen, chief research analyst at the IPO Value Monitor. Already, Juniper has lined up an impressive list of blue-chip clients, including AT&T, IBM Global Services, UUNet (a unit of MCI WorldCom), Exodus Communications, Qwest Communications, and Ericsson.

That record was more than enough to impress investors. Juniper's IPO opened on June 25 at $34 a share, then shot up 191%, to $99, by day's end. That gave the company an instant market cap of $4.9 billion--the third-highest market cap ever after one day of trading, according to Thomson Financial Securities Data.

Still, Juniper has a long way to go to rival Cisco, whose market cap is $211 billion. And relying on one product has risks. For instance, once Cisco comes out with its new router, Tuen points out, "anything Cisco does could really hurt Juniper."

Juniper and Cisco actually have a long history together. Juniper CEO Scott Kriens was one of the co-founders of StrataCom, acquired by Cisco in 1996 for about $4 billion. And Tony Li, one of the main architects of Juniper's router, is a former Cisco engineer.

--Jeanne Lee