Lexmark IBM 9077 manual Supported Routing Protocols, Subnet Masking/Supernetting

Subnet Masking/Supernetting

Variable length subnet masking is a classless addressing scheme for interdomain IP packet routing. It is a way to more efficiently manage the current 32-bit IP addressing method. Subnet masks let sites configure the size of their subnets based on the site needs, not on the arbitrary Class A, B, and C structure originally used in the Internet addressing. Class-based addressing restricts the boundary to the 8-bit boundaries and is implicitly derived from the first eight bits of the network address. The new addressing method allows the network portion of an IP address to be separated from the host portion of the address at any point within the 32-bit address space. This expanded boundary is called the "netmask" and is explicitly provided to the router along with the network address information. Class-based addressing restricts the boundary to the 8-bit boundaries and is implicitly derived from the first eight bits of the network address.

Subnet masking offers a number of benefits by extending the current address space. By eliminating implicit netmask assignments, addresses can now be assigned from any unused portion of the entire 32-bit address range rather than from within a specific subset of the space previously called a class. Since it hides multiple subnets under a single network address, this method is called supernetting.

Classless addressing allows the network administrator to further apportion an assigned address block into smaller network (or host) segments based on powers of two (2, 4, 8, 16 networks, for example). Knowledge of the apportioned segments need not be communicated to exterior peers. They need only a single pointer to the entire address block. Not only does subnet masking better utilize address space, but implemented properly it results in significantly smaller routing tables.

2.2.2 Supported Routing Protocols

In the days of a single Internet, core groups of independent networks were called autonomous systems. We will use the term autonomous systems (AS) in the following description of protocols. The routing protocols supported on the GRF can be divided into two classes: Interior routing protocols or interior gateway protocols (IGPs) and Exterior routing protocols (EGPs).

•Interior routing protocols

Interior routing protocols are used to exchange routing information between routers within a single autonomous system. They are also used by routers that run exterior protocols to collect network reachability

20IBM 9077 SP Switch Router: Get Connected to the SP Switch