TCP/IP Tutorial and Technical Overview - IBM Redbooks

3.1.9 IP datagram
Therefore, a larger proportion of the network costs are incurred by the
receiving organization.
Because multihomed domains vary greatly in character, none of the these
schemes is suitable for every domain. There is no single policy that is best.
RFC 1518 does not specify any rules for choosing between them.
CIDR implementation
The implementation of CIDR in the Internet is primarily based on Border
Gateway Protocol Version 4 (see 5.9, “Border Gateway Protocol (BGP)” on
page 215). The implementation strategy, described in RFC 1520, involves a
staged process through the routing hierarchy beginning with backbone routers.
Network service providers are divided into four types:
► Type 1: Those providers that cannot employ any default inter-domain routing.
► Type 2: Those providers that use default inter-domain routing but require
explicit routes for a substantial proportion of the assigned IP network
numbers.
► Type 3: Those providers that use default inter-domain routing and
supplement it with a small number of explicit routes.
► Type 4: Those providers that perform inter-domain routing using only default
routes.
The CIDR implementation began with the Type 1 network providers, then the
Type 2, and finally the Type 3 providers. CIDR has already been widely deployed
in the backbone and more than 190,000 class-based routes have been replaced
by approximately 92,000 CIDR-based routes (through unique announced
aggregates).
The unit of transfer in an IP network is called an IP datagram. It consists of an IP
header and data relevant to higher-level protocols. See Figure 3-16 for details.
header data
base IP datagram...
physical network header IP datagram as data
encapsulated within the physical network's frame
Figure 3-16 IP: Format of a base IP datagram
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