 - A fixed-length subnet mask (FLSM) is a sequence of numbers of unchanging length that streamlines packet routing within the subnets of a proprietary network. A subnet can be a geographically defined local area network (LAN). Alternatively, a subnet may define security boundaries, departmental boundaries, multicast zones or hardware security parameters.
Once a packet has arrived at an organization's main gateway with its network number, that packet is routed to its ultimate destination using a subnet number. The FLSM is usually a string of binary digits shown over the subnet number, telling the router which parts of the subnet number to look at. A binary "1" over a particular digit in the subnet number says "Pay attention to this digit." A "0" says "Ignore this digit."
The use of an FLSM saves a router the task of having to handle an entire IP address because the router deals only with the digits selected by the mask. Nevertheless, this method of subnet masking can result in inefficient use of IP address space because subnets are rarely filled to capacity. In networks having many unassigned IP addresses, a variable-length subnet mask (VLSM) can provide more efficient use of address space.
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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: |
22 May 2007
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