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IPv4 Address Classes

Troubleshooting IP Routing Protocols offers you a full understanding of invaluable troubleshooting techniques that help keep your network operating at peak performance. Whether you are looking to hone your support skills or to prepare for the challenging CCIE exams, this essential reference shows you how to isolate and resolve common network failures and to sustain optimal network operation.


IP addressing is central to the operation of the IP protocol. The TCP/IP stack features a network interface to the underlying physical data-link layers, which allow the IP protocol to be media independent. Media independence is probably one of the critical advantages of the IP protocol that has promoted its wide acceptance and ubiquity. IP uses a native addressing scheme, in line with its media-independent architecture, that has no bearing on the underlying local-area network (LAN) or wide-area network (WAN) media interconnect IP devices. Therefore, IP successfully operates over heterogeneous network infrastructures consisting if several kinds of different media technology. This flexibility, together with a simple protocol stack, is the most critical instigator of its popularity.

IP addressing assigns addresses to individual network interfaces of a device (link-based approach) instead of using a single address for the whole device (host-based approach). The various interfaces are connected to network links that are designed as subnetworks (or subnets) and are assigned subnet addresses. An interface's IP address is assigned from the subnet address space of the connecting link. The advantage of this link-based addressing approach is that it allows routers to summarize routing information by keeping track of only IP subnets in the routing tables instead of every host on the network. This is advantageous especially for broadcast links such as Ethernet that might have many devices connected at the same time. The Address Resolution Protocol (ARP) is used in IP networking for resolving the IP addresses of directly connected hosts to the corresponding data-link addresses.

Currently, two types of IP addresses exist: IP Version 4 addresses (IPv4) and IP Version 6 addresses (IPv6). IPv4 addressing, which was in place before IPv6 was adopted, uses 32 bits to represent each IP address. This 32-bit addressing scheme provides up to 2^32 (4,294,967,295) unique host addresses, mathematically speaking. With the ever increasing size of the global Internet, the 32-bit IPv4 addressing scheme has turned out to be insufficient for the foreseeable future, prompting the introduction of the 128-bit IPv6 addressing scheme. This book covers troubleshooting of IP routing protocols deployed in IPv4 environments. Therefore, the ensuing discusses only the IPv4 addressing structure and related concepts, most of which are applicable to IPv6. The following IPv4 addressing topics are covered in the subsequent sections:

IPv4 address classes
Private IPv4 address space
IPv4 subnetting and variable-length subnet masking
classless interdomain routing

IPv4 Address Classes

As explained in the previous section, the 32-bit IPv4 addressing scheme allows a large number of host addresses to be defined. However, the link-based addressing scheme adapted by IP requires network links to be associated with groups of addresses from which the connected hosts are assigned specific addresses. These address groups, described also as address prefixes, are referred to in classical IP terminology as IP network numbers.

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