IP addressing is central to the operation of the IP protocol. The TCP/IP stack shown in Figure 1-1 features a network interface to the underlying physical and 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 of 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 of a device are connected to network links that are designated 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 232 (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 practical troubleshooting of IP routing protocols deployed in IPv4 environments. Therefore, the ensuing text 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
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 adopted 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.
Originally, IP network numbers were defined with rigid boundaries and grouped into ad-dress classes. The idea behind IP address classes was to enable efficient assignment of the IP address space by creating address groups that would support a varying number of hosts. Network links with fewer hosts then would be assigned an address from a class that sup-ports an appropriate number of attached hosts. Another benefit of address classes was that they helped streamline the address-allocation process, making it more manageable.
Five address classes? A, B, C, D, and E? were defined and distinguished by the setting of the most significant bits of the most significant byte in the IP address. Each address class embraced a set of IPv4 address subnets, each of which supported a certain number of hosts. Table 1-1 shows the five IPv4 classes.
Table 1-1. IP Address Classes and Representation
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