HOSTS is a text file that contains a local host lookup table. When the stack attempts to resolve a host name to an IP address, it firsts look in the HOSTS file. If a matching entry exists, it will use that entry. If an entry does not exist, it will proceed to use DNS.
Here is an example HOSTS file:
# This is a sample HOSTS file
caesar 192.168.0.4 # Server computer
augustus 192.168.0.5 # Firewall computer
The host name and IP address can be separated by spaces or the tab character. Comments can also be inserted into the file by preceding the line with a # character. HOSTS can be used to fake entries, or override DNS entries. The HOSTS file is often used on computers on a small LAN that have no DNS server. The HOSTS file is also useful for overriding host
IP addresses for debugging. You do not need to read the HOSTS file; the stack will take care of this detail for you transparently whenever name resolution is necessary.