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CCIE Security Exam Certification Guide by Henry Benjamin

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Variable-Length Subnet Masks

A variable-length subnet mask (VLSM) is designed to allow greater use of IP address space by borrowing bits from the subnet mask and allocating them to host devices. To allow a greater number of devices to connect to the Internet and intranets, the standards body of various routing protocols designed an IP routing algorithm to cater to IP networks with a different subnet mask than the default used in classful networks.

NOTE

Routing algorithms that support VLSM are as follows:

  • RIP Version 2

  • OSPF

  • IS-IS

  • EIGRP

  • BGP4

Additionally, Cisco IOS allows the use of any 0 subnets (for example, subnet 131.108.0.0/24) with the global IOS command, ip subnet-zero. This can be very useful for networks running out of IP address space.

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