Cabletron Systems DMS-100 - Manual (Page 160)

160 Appendix E: Understanding IP and IP addressing
297-8991-910 Standard 03.01 August 1999
representative to discuss state-of-the-art secure data communications
equipment products.
Variable-width subnetworks
When subnetworks were first invented, they were intended to be used in a star
topology, with the major router at the port of entry connected to all
subnetworks. All subnetworks were supposed to have address ranges of the
same size. Later IP implementations have retained the expectation that the
width of the subnet mask is uniform throughout all the subranges of the top
level (class A or B) network number.
With the current increased emphasis on conservation of IP addresses, it is often
desirable to allocate subnet ranges of “just the right size”. To allocate ranges
consistently, all protocol exchanges that communicate a network address range
must include the associated subnet mask. OSPF performs this, and future
releases of other route information protocols (for example, RIP Version 2)
allow this too. On the other hand, some protocols do not carry this information,
since knowledge of subnet structure is contained within a routing domain, and
is invisible outside of a routing domain.
It is important to realize that support for variable-width subnetworks does not
allow for subnetworking subnetworks. For example, an IP port can have the
class B address 129.191.14.1 with subnet mask 255.255.255.0. IP address
129.191.0.0 is the network, and IP address 129.191.14.0 is the subnet. You
cannot further subnet the 129.191.14.0 subnet—for example, 129.191.14.128
with mask 255.255.255.128. However, a new subnet can be created with a
longer mask, such as 129.191.15.128 with mask 255.255.255.128.
Protocols related to Internet Protocol
This section provides brief descriptions on the constituent protocols of TCP/IP.
Internet Protocol
IP is a connectionless datagram service that provides the following benefits:
• best-effort delivery
• internetwork-wide addressing
• fragmentation and reassembly
• time-to-live control of datagrams
• checksum verification of header contents
IP is defined in RFC791.