(SUBNETwork mask) The technique used by the IP protocol to create a subnet address. The subnet mask is a binary pattern that is stored in the client machine, server or router. It is matched with the IP address of a packet to determine which network segment the packet is destined for. See subnet.
A Tradeoff Between Hosts and Subnets
Depending on the network class (A, B or C), some number of bits are reserved for hosts and subnets, and these bits become a tradeoff. The more hosts, the fewer the subnets can be created; the more subnets, the fewer the hosts can be individually addressed.
A Subnet Mask Example
The subnet mask below is a Class C address, which uses the first 24 bits for network ID and the last 8 for host ID. These last 8 can be divided between hosts and subnets. In the default mask below, the 0 means there are no subnets and up to 254 hosts can be addressed.
In the second example, the 224 reserves the three high-order bits of that field for subnets, leaving the remaining 5 bits for hosts. This 224 pattern creates subnets in the range of 001 to 110 (000 and 111 are reserved). As such, 6 subnets of up to 30 hosts in the range of 00001 to 11110 can be addressed. Once again, 00000 and 11111 are reserved: all zeros mean "this" node, and all ones mean "all" nodes (broadcast), which is why calculations for maximum hosts and subnets are always subtracted by 2. For a list of all Class B and C subnet masks, see subnet mask tables.
A Tradeoff Between Hosts and Subnets
Depending on the network class (A, B or C), some number of bits are reserved for hosts and subnets, and these bits become a tradeoff. The more hosts, the fewer the subnets can be created; the more subnets, the fewer the hosts can be individually addressed.
A Subnet Mask Example
The subnet mask below is a Class C address, which uses the first 24 bits for network ID and the last 8 for host ID. These last 8 can be divided between hosts and subnets. In the default mask below, the 0 means there are no subnets and up to 254 hosts can be addressed.
In the second example, the 224 reserves the three high-order bits of that field for subnets, leaving the remaining 5 bits for hosts. This 224 pattern creates subnets in the range of 001 to 110 (000 and 111 are reserved). As such, 6 subnets of up to 30 hosts in the range of 00001 to 11110 can be addressed. Once again, 00000 and 11111 are reserved: all zeros mean "this" node, and all ones mean "all" nodes (broadcast), which is why calculations for maximum hosts and subnets are always subtracted by 2. For a list of all Class B and C subnet masks, see subnet mask tables.
Class C Default Submask 255.255.255.0 11111111.11111111.11111111.00000000 Class C Subnet Mask (6 Subnets/30 Hosts Each) 255.255.255.224 11111111.11111111.11111111.11100000
 | Reproduced with permission from Computer Desktop Encyclopedia. Copyright (c) 1981-2009 The Computer Language Company Inc. All rights reserved. |
Additional Resources
- IP Calc 1.0 (Mobile)
- Enter the desired IP Address, then use the slider to select your subnet mask and every subnet related item is immediately calculated and displayed on one simple screen. IP Calc features instant mobile CIDR IPv4 calculations, displays network and broadcast addresses, subnet and Cisco Wildcard masks, the host range (HostMin...
- Software downloads 2009-06-25
- IPViewer 1.5 (Windows)
- IPViewer allows users to view their IP address, subnet mask, and host name of their computer. You can park the application in the system tray or stick it above all other apps on the desktop. You can also copy and paste your ip address and host name to the clipboard....
- Software downloads 2009-06-24
- IP Shifter 2.3.5 (Windows)
- IP Shifter is a smart software tool that helps you to change your TCP/IP related settings easily and quickly. If you work dynamically at several locations in the network due to business, you need to be connected in different offices, or you need to use your computer in more than...
- Software downloads 2009-06-20
- Newt Professional 2.5 Build 139 (Windows)
- A network inventory & discovery tool providing fast and easy scanning of over 200 items from NT-based machines with no user-deployment time. Gathered information is displayed in an easy-to-read spreadsheet-like format. Features include discovery & scanning by domain, workgroup, IP range, or by using the built-in network browser featuring real-time...
- Software downloads 2009-06-14
- IP Address Menu 5.2 (Mac)
- You want to display your current IP address in the menu bar? You want to display your user name, your computer name, the IP address, the subnet mask and the MAC address of each of your network interface at the same time? You want your Mac to automatically send an...
- Software downloads 2009-05-23
- SwitchIP 3.5.6 (Windows)
- SwitchIP Address software allows you to modify the IP address of your local machine with a single click. Especially handy for laptop users who travel between sites and need to constantly switch IP address, gateway address, subnet mask. With switchIP you don't waste your time reconfiguring computer network settings every...
- Software downloads 2009-04-22
- NetSetMan 2.5.4 (Windows)
- NetSetMan is a network settings manager which can easily switch between 6 different profiles including IP address, subnet mask, default gateway, DNS server, Win server, computer name, printer, DNS domain, workgroup, and scripts. It also can get current settings and do a FastSwitch from the tray bar. With only two...
- Software downloads 2009-04-02
- Beware of the TrixBox
- For those of you thinking of cutting over to a packaged Asterisk solution, you'll want to read this account from Dr. Adrian Steele, the Managing Director of Mercian Labels, a UK SME specialising in short run label printing and...
- Blog posts 2009-02-09
- 40ò0of parents feel that writing on a computer makes their child more likely to use poor spelling and grammar
- 40ò0of parents feel that writing on a computer makes their child more likely to use poor spelling and grammarAgreed.Any time you let a machine do for you something you should be doing, whether it grammar, spelling, or in my case as a CPA letting a machine add a column of...
- Discussion threads 2008-06-29
- IPMaster 1.4 (Windows)
- IPMaster is a IP address management software. It provides visual IP address assigningnetwork topology tree, automatic subnet calculation, mask calculation, subnetting, network segment scanning, host monitoring, ping, traceroute, telnet, and netsend. It supports VLSM and CIDR. The purpose of this software is to assign and manage IP address efficiently for...
- Software downloads 2008-05-26
- Whatmask 1.2 (Windows)
- Whatmask runs in a DOS shell Command Prompt. Whatmask is a small C program that will help you with network settings. Whatmask can work in two modes. The first mode which is how prior versions worked is to invoke Whatmask with only a subnet mask as the argument. In...
- Software downloads 2007-12-08
- Free IP Switcher 1 (Windows)
- Whenever mobile user moves to a different network, he needs to change particular IP address, gateway address, subnet mask, proxy server, and default printer to particular network environment. In order to do it, he needs to remember TCP/IP settings information of every network. Also, he has to reboot the computer...
- Software downloads 2007-11-28
- NetSetting 1.02 (Windows)
- NetSetting is a small program for creating and managing profiles for TCP/IP protocol setting (IP address, subnet mask, default gateway and DNS servers). The user can create profiles for every network adapter and then use these profiles as needed. The program goes into the tray after start, so it is...
- Software downloads 2007-10-30
- Windows CE Platform Builder 2.12 QFE 31 QFE31 (Windows)
- Fixes made in this QFE:Components: DHCPDescription: Windows CE 2.12 DHCP client fails when receiving a DHCPACK message with a subnet mask, but the previous DHCPOFFER didn't send such information. This version is the first release on CNET Download.com.
- Software downloads 2007-09-14
- Vonage floats new voice gateway test offer: they'll pay you, too!
- TechCrunch's Michael Arrington reports he has received an email from Vonage inviting him to test a new gateway device that will, according to the email, "inspire" consumers to proudly display their Vonage router rather than hide it under their desk."rnrnIf you are looking for this new device on the Vonage...
- Blog posts 2007-08-03
- IP Changer 0.2 (Windows)
- IP Changer allows you to change the IP Address SubNet Mask Gateway DNS Server DHCP Status of your LAN Interface. You can save a set of settings for future usage or if you are frequently changing sites using your Laptop and do not want to remember for each site the...
- Software downloads 2007-05-10
- Reliable Subnet Calculator 1.1 (Windows)
- Reliable Subnet Calculator is a security solution that allows one to quickly calculate subnet mask for a given IP address range. Necessity to accurately calculate subnet mask can be a very tedious task for individuals lacking thorough understanding of IP addressing, subnetting and other networking concepts. Reliable Subnet Calculator is...
- Software downloads 2006-10-28
- LeeT Network Information 1 (Windows)
- This software is for Windows XP only. It gathers information about your personal computer and your network at very fast speeds. It grabs your operating system, serial number, operating system version, memory chip/s, CPU type, CPU clock speed, LAN address, WAN address, subnet mask, & MAC address. This is a...
- Software downloads 2006-07-20
- Easy way to learn IP subnetting
- Easy way to learn IP subnettingMost ExcellentThanks George.Win2K ignores Mask to reach GatewayMicrosoft Windows 2000 ignores the Subnet Mask to reach the Gateway.We host our web servers at an ISP. The original server is running Microsoft Windows 2000. Several years ago, we added a Linux server. I...
- Discussion threads 2006-06-28
- LanCalculator 1.0.2 (Windows)
- LanCalculator - makes possible not only to easily calculate the subnet addresses range and subnet masks but also broadcast address, net address, net prefix and net mask inversion which is used in the access control lists ACL of Cisco network equipment. One key press is enough to calculate all parameters...
- Software downloads 2006-06-06
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