Networking Tutorial (TCP/IP Routing)

CIDR (Classless Inter-Domain Routing)

From this information, the Operating System can determine the most useful router to choose for a particular destination. On Solaris, the /etc/netmasks file tells the OS about particular netmasks for given networks; otherwise, the old, pre-CIDR standard is followed, whereby the IP address itself suggests its netmask:

You can see that each class (A,B,C) has a "Private" segment in the middle, which is non-routable. Other than that, their netmasks are 255.0.0.0, 255.255.0.0. and 255.255.255.0 (ff000000, ffff0000, ffffff00 respectively, in Hex). That turned out to be a little too simplistic as internet usage grew, so we now have Classless Inter-Domain Routing (CIDR), which forgets about classes, and just says that a network can have any netmask. The closer you get to such a network, the more likely you are to need to know about how it is configured (hence /etc/netmasks, and CIDR in DNS, etc).