Encyclopedia

subnet mask

Also found in: Dictionary, Medical, Acronyms, Wikipedia.

subnet mask

This article is provided by FOLDOC - Free Online Dictionary of Computing (foldoc.org)

subnet mask

(SUBNETwork mask) The technique used by the TCP/IP communications protocol that identifies which network segment a packet belongs to. The subnet mask is a binary pattern, and the default mask found in countless small local networks indicates that all the machines are in the same network (see below). See subnet, IP address and TCP/IP.

A Tradeoff Between Hosts and Subnets
The bits in the mask identify both hosts and subnets. The more hosts, the fewer subnets; the more subnets, the fewer hosts can be individually addressed. These bits become a tradeoff based on the network class (A, B or C).

Class C Subnet Mask Examples
Subnet masks for a Class C network (small network) use the first 24 bits for subnet ID and the last 8 for host ID. These last 8 are divided between hosts and subnets (for more on network classes, see subnet mask tables).

In Example #1 below, the 255.255.255 are the numeric values of three sets of eight 1 bits. The 0 means eight 0 bits. There are "no" subnets, and up to 254 hosts can be addressed in this network (255 minus 1).

Example #1Class C Default Mask (No Subnets)255.255.255.0
 11111111.11111111.11111111.00000000


 Example #2Class C Mask for Six Subnets255.255.255.224
 11111111.11111111.11111111.11100000
                            ssshhhhh

In Example #2 above, the 224 reserves the three high-order bits of that byte for subnets (sss), leaving the remaining five bits for hosts (hhhhh). The 224 creates six subnets from 001 to 110 (000 and 111 are reserved), and each subnet can have 30 hosts from 00001 to 11110. Likewise, 00000 and 11111 are reserved: 00000 means "this" node, and 11111 means "all" nodes (see broadcast address). This is why calculations for maximum hosts and subnets are always minus 2.
Copyright © 1981-2025 by The Computer Language Company Inc. All Rights reserved. THIS DEFINITION IS FOR PERSONAL USE ONLY. All other reproduction is strictly prohibited without permission from the publisher.
Mentioned in
References in periodicals archive
In the architecture of DHCP, the DHCP server is responsible for maintaining the IP address pool and other network parameters, such as subnet mask and default DNS.
Firstly, comparing to RIP, OSPF is a classless protocol which allows utilization of different subnet masks, which essentially gives network administrators more flexibility with IP addresses and less wastage.
Next we prepare the network devices, which need two pieces of information to communicate: an Internet protocol (IP) address on the network (four sets of numbers) and a subnet mask. Some network devices are configured automatically, but doing it manually is not difficult, although, as you'll see, it involves many steps.
* The destination prefix (an IP address plus subnet mask; for instance 23.154.56.0/24, which represents 256 IP addresses).
The range 0-63 cannot be used with that subnet mask, since in that case the subnet field would consist of all zeroes.
To determine whether the destination device is on the local subnet or on a remote one, i.e., whether the message may be sent to the destination device directly or must be sent to the IP gateway, the originating device uses a configuration parameter called the "subnet mask," AND's it with the source and destination IP addresses, and compares the results.
In TCP/IP, a subnet mask separates the network ID from the host ID.
It also automatically configures itself, including selecting an appropriate IP address and subnet mask, which allows me to have a `virtual presence' anywhere on the network.
Copyright © 2003-2025 Farlex, Inc Disclaimer
All content on this website, including dictionary, thesaurus, literature, geography, and other reference data is for informational purposes only. This information should not be considered complete, up to date, and is not intended to be used in place of a visit, consultation, or advice of a legal, medical, or any other professional.