What is the subnet mask used for?

[tarak]

What is the subnet mask used for?

[Will.Spencer]

The subnet mask is used by your TCP/IP stack to decide where to send IP packets.

TCP/IP uses the subnet mask to determine if a destination address is inside your local network, or if the destination address can only be reached through a router.

If the destination IP address is on the local network, the TCP/IP stack tells your network card to send the data packets directly to the destination.

If the destination IP address is not on the local network, the TCP/IP stack tells your network card to send the data packets to a router.

We have a free web tool which you can use to answer the question "Are these two IP addresses on the same network?"

This tool asks for two IP addresses and the subnet mask. If the subnet mask would place these two on the same network, the tool outputs the message "These two IP's are on the same network." This helps you to see the results of using different subnet masks.