I have a cisco router with 4 wires 56/64k dsu/csu network adapter. what kind of modem do I need to connect to this 4-wires 56/64k dsu/csu network adapter? this is a cisco 2525 model.

Okay...based on what you have written you most likely have a 1-port serial w/4-wire 56/64Kbps interface module. On top of that, a 2525 is a "token ring" not "ethernet" router. What are the odds you are using Token Ring in your network right now...very very little. If you don't even have a token ring NIC in your PC or PCs the discussion as to what modem to use becomes real mute real quick. But if you must know... There is no modem you can use. A dsu/csu IS A MODEM for a serial connection - a serial connection is a T1 line or Fractional T1 line. That is for synchronus data. A modem is used for "Asynchronus". Therefor you don't need a modem you need a T1 or Fractional T1 line. You can't use DSL, you can't use a Cable modem, you can't use a regular phone line, you can only use a T1 flavor line. On top of all this, Cisco no longer even supports "2500s" so I would advise against you using it in a business. Please tell me that you got the router for free, or as a gag gift, cause brotha the only thing you can do with it now is use it as a door stop. I doubt you'd even get eBay action on it. D

thanks a lot for explaining. I got it off ebay. Can I use the "aux" port? and which kind of modem I can use?

Highly unlikeyly. The Aux port is a control and command port designed for remote access. Unlike a standard port, it is designed to give you access "into" the router, but not "thru" the router as is a standard serial port is designed for. In addition you don't have Token Ring set up on your internal network trying to use the AUX port become a mute point. If you were to use the Aux port for remote access, any standard external modem will work. The route I would choose in this situation is to return the item to the eBay seller and get your money back. You'll take a hit from eBay, but it's better than using a router you won't be using. If you want a standard router that works with a modem I can get you that at probably the price you paid or less for the 2525. Even if it's Cisco. Send me an email. D

Okay, I can admit when I'm wrong. I went and queried some folks cause I thought it was an interesting enough question, and turns out you can config the Aux port to work as a serial port. There are some limitations (and hacks) but the answer would still be yes. D