 - A subnet mask is a screen of numbers used for routing traffic within a subnet. Once a packet has arrived at an organization's gateway or connection point with its unique network number, it can be routed to its destination within the organization's internal gateways using the subnet number. The subnet mask consists of a screen of numbers indicating to the router which numbers it should look at underneath. In a binary mask, for example, a "1" over a number says "Look at the number underneath;" a "0" says "Don't look." Using a mask saves the router having to handle the entire 32 bit address; it can simply look at the bits selected by the mask.
Using the previous example (which is a very typical case), the combined network number and subnet number occupy 24 bits or three of the quads. The appropriate subnet mask carried along with the packet would be:
255.255.255.0
Or a string of all 1's for the first three quads (telling the router to look at these) and 0's for the host number (which the router doesn't need to look at). Subnet masking allows routers to move the packets on more quickly.
If you have the job of creating subnets for an organization (an activity called subnetting) and specifying subnet masks, your job may be simple or complicated depending on the size and complexity of your organization and other factors.
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Learn more about IP Networking |
| How to subnet: Subnetting calculations and shortcuts: Learn how subnetting works in this tip and quiz on how to calculate a subnet mask. IP address classes and bit values of octets are explained in detail. |
| IP addressing and subnetting explained: Our IP addressing and subnetting crash course provides an overview of IP addressing-related topics, including IPv4 and IPv6, subnetting, DHCP, and calculating subnet masks. |
| Troubleshooting IP Routing -- 'CCNA Official Exam Certification Library, 3rd Edition,' Chapter 7: Learn how to troubleshoot IP routing in preparation for the Cisco CCNA exam in this chapter from 'CCNA Official Exam Certification Library, 3rd Edition.' |
| IP address management -- from 'Network troubleshooting and diagnostics': Learn how IP address management and maintenance tools can help manage the scope of IP addresses on your network. |
| IP Accounting -- from Network Management: Accounting and Performance Strategies: IP Accounting, Chapter 6 of Network Management: Accounting and Performance Strategies, describes the IP Accounting features and functions in Cisco IOS, and SNMP MIB. |
| Basic IP Connectivity and CEF Troubleshooting -- Chapter 4 of "Cisco Express Forwarding": This chapter presents the general troubleshooting used on Cisco IOS routers and switches as a first step in troubleshooting IP connectivity problems. |
| LAST UPDATED: |
21 Mar 2007
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