How do I setup a HOSTS file on different operating systems?

Hosts file on Linux/Unix:

/etc/hosts

ip.nu.mb.er   name1 name2

Hosts file on Windows:

%WINDOWS%\System32\Drivers\etc\hosts (e.g. C:\Windows\System32\Drivers\etc\hosts)

ip.nu.mb.er   name1 name2

Assuming you're asking how to find the file (it will most likely exist already):

• In a Unix-like operating system (including Mac OS X and Linux), you'll typically find the hosts file in /etc/hosts. Note that Mac OS X does not by default show you the etc directory. You can show it in various ways, such as by choosing Go → Go to Folder… in Finder, and typing /etc.
• In an NT-based operating system (including Windows XP and Windows Vista), the etc directory is slightly hidden, in %WINDIR%\system32\drivers\, e.g. C:\WINDOWS\system32\drivers\etc\hosts.