The hosts file is a computer file used in an operating system to map hostnames to IP addresses.
The hosts file is a plain-text file and is traditionally named hosts.
No wildcards are allowed and only the main domain name is a valid entry.
1. Host file is loaded into memory (cache) at startup,
2. then windows checks it before it queries any dns servers, which enables it to override addresses in the dns.
Eg: to host file at:
c:\WINDOWS\system32\drivers\etc\hosts
Add following entry to direct url dev.andrewsin.com to a server
83.111.111.111 dev.andrewsin.com
Eg2: loopback:
This entry is a not an actual Internet IP address but defines a local address and can be used to direct the computer to send a packet to itself.
127.0.0.1 localhost
Blocking sites:
By overriding addresses, you can prevent access to sites by redirecting any connections back to the your machine (local machine).
Eg: By adding following entry to hosts file:
It blocks all files supplied by that DoubleClick Server to the web page you are viewing.
127.0.0.1 ad.doubleclick.net
For more details on blocking sites, see here.
