January 19, 2012

1.3.2 Subnetting


Subnetting is how networks are divided. RFCs 1466 and 1918 detail subnetting and can be found at http://www.faqs.org/rfcs/.

The practice of dividing a single network into two or more networks is called subnetting and the networks created are called subnetworks or subnets.

This results in the logical division of an IP address into two fields, a network or routing prefix and the host identifier.

The routing prefix is expressed in CIDR notation. It is written as the first address of a network, followed by a slash character (/), followed by the bit-length of the prefix. For example, is the prefix of the IPv4 network starting at the given address, having 24 bits allocated for the routing prefix, and the remaining 8 bits reserved for host addressing.

In IPv4 the routing prefix can also be specified in the form of the subnet mask, expressed in quad-dotted decimal representation, e.g. is the network mask for the prefix.

If definitions are helpful to you, use these vocabulary terms to get you started:
  • Address—The unique number ID assigned to one host or interface in a network.
  • Subnet—A portion of a network sharing a particular subnet address.
  • Subnet mask—A 32-bit combination used to describe which portion of an address refers to the subnet and which part refers to the host.
  • Interface—A network connection.
The smallest subnet that has no more subdivisions within it is considered a single "broadcast domain," which directly correlates to a single LAN (local area network) segment on an Ethernet switch.

Subnets have a beginning and an ending, and the beginning number of a specific subnet is always even ( and the ending number is always odd ( The beginning number is the "Network ID" and the ending number is the "Broadcast ID".

Subnetting an IP Network can be done for a variety of reasons, including traffic segmentation, organization, preservation of address space, and security. In an Ethernet network, all nodes on a segment see all the packets transmitted by all the other nodes on that segment. Performance can be adversely affected under heavy traffic loads, due to collisions and the resulting retransmissions.

The subnet mask plays a crucial role in defining the size of a subnet, limiting broadcast traffic to within the subnet and hiding network details from external users.

Subnetting for IPv4 was originally defined to make better use of the host bits for Class A and Class B IPv4 public address prefixes.

  • http://en.wikipedia.org/wiki/Subnetwork
  • http://www.techrepublic.com/article/ip-subnetting-made-easy/6089187
  • http://technet.microsoft.com/en-us/library/bb726997.aspx
  • http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a00800a67f5.shtml
  • http://www.ralphb.net/IPSubnet/subnet.html
  • CompTIA Security+ Study Guide: Exam SY0-301, Fifth Edition by Emmett Dulaney

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