How would you find someones isp?

Answer 1

You need neither a tracer, nor a sniffer. Standard Windows (or Mac OS) will easily do. However, you do need some form of a network address for the target organization, either a web address (www.xyz.com) or an IP address.

Do a "traceroute" to that address (in Windows, open a command box and type "tracert xxx.xxx.xxx.xx", replacing the "xxx" with the party's address; in Mac OS, the command is "traceroute xxx.xxx.xxx.xxx").

You will be seeing a list of "hops", starting with your own address, and ending with the target address. The last hop in the list is the target address. The ISP's address is the last one in the list before the trace route reaches the target. If the target is a home user, then the ISP's address is likely the second to last. If the target is part of an organization (e.g. a school or a company), then the second to last address is likely a gateway address, and the ISP's router address is the third-to-last address in the list.
Typically, the host names give that away (look for names like ("xyzGW") to indicate gateways.
Once you have a designation for the ISP's router, you can either get the name directly, or you might have to do a "whois" lookup (google for "whois") to identify who is behind the router name.

Hope this helps!

P.S.: comment on Kutti's answer: running a "whois" on the target address will give you their hosting service, not their ISP. The two maybe identical, but often they are not.

Answer 2

If you have a sniffer, you could get the ip address....once you have the address you could visit the link below and find the isp.....

http://visualroute.visualware.com/

Update: You said
"You need neither a tracer, nor a sniffer. Standard Windows (or Mac OS) will easily do. However, you do need some form of a network address for the target organization, either a web address (www.xyz.com) or an IP address."

Of course you need to know an ip address or web address to do a trace route, but if you dont have that, you need a program to sniff the address idiot.....

Answer 3

Use the whois command if you are on a *nix system or use the following link,

http://networking.ringofsaturn.com/Tools/whois.php

If 'whois' doesn't give you the required info, try doing a traceroute to that ip and if you are lucky you will get the ips in the path which can be used in turn on whois to get close to the isp information.

Answer 4

Find the ip address, Use an IP tracer and voila, ISP found

Answer 5

if you want to get it online, you can use either a paid, or a free service such as GeoIP, a free demo is available at http://www.maxmind.com/app/locate_ip. it will tell you the country, city, post code, organization name, ISP, and a whole lot of other information. there's also another one at http://www.ipligence.com/geolocation/?lang=en&search#, but i'd prefer to use the first one

Answer 6

If you know the domain or website address, look into dnsstuff.com