One of the earliest difficulties encountered when the Internet began to grow was the inconvenience of having to allocate a new network number for any new network segment that was to be attached to the Internet. This became especially cumbersome with the development and increasing use of local area networks (LANs) in the early 1980s. To address the problem, it was natural to consider a way that a site attached to the Internet could be allocated a network number centrally that could then be subdivided locally by site administrators. If this could be accomplished without altering the rest of the Internet’s core routing infrastructure, so much the better.
The subnet mask is an assignment of bits used by a host or router to determine how the network and subnetwork information is partitioned from the host information in a corresponding IP address. Subnet masks for IP are the same length as the corresponding IP addresses (32 bits for IPv4 and 128 bits for IPv6). They are typically configured into a host or router in the same way as IP addresses—either statically (typical for routers) or using some dynamic system such as the Dynamic Host Configuration Protocol (DHCP; see Chapter 6). For IPv4, subnet masks may be written in the same way an IPv4 address is written (i.e., dotted-decimal). Although not originally required to be arranged in this manner, today subnet masks are structured as some number of 1 bits followed by some number of 0 bits. Because of this arrangement, it is possible to use a shorthand format for expressing masks that simply gives the number of contiguous 1 bits in the mask (starting from the left). This format is now the most common format and is sometimes called the prefix length. Table 2-4 presents some examples for IPv4.
So far we have discussed how a network number allocated to a site can be subdivided into ranges assigned to multiple subnetworks, each of the same size and therefore able to support the same number of hosts, based on the operational expectations of the network administrator. We now observe that it is possible to use a different-length subnet mask applied to the same network number in different portions of the same site. Although doing this complicates address configuration management, it adds flexibility to the subnet structure because different subnetworks may be set up with different numbers of hosts. Variable-length subnet masks (VLSM) are now supported by most hosts, routers, and routing protocols. To understand how VLSM works, consider the network topology illustrated in Figure 2-5, which extends Figure 2-3 with two additional subnetworks using VLSM.
In each IPv4 subnetwork, a special address is reserved to be the subnet broadcast address. The subnet broadcast address is formed by setting the network/subnetwork portion of an IPv4 address to the appropriate value and all the bits in the Host field to 1. Consider the left-most subnet from Figure 2-5. Its prefix is 128.32.1.0/24. The subnet broadcast address is constructed by inverting the subnet mask (i.e., changing all the 0 bits to 1 and vice versa) and performing a bitwise OR operation with the address of any of the computers on the subnet (or, equivalently, the network/subnetwork prefix). Recall that the result of a bitwise OR operation is 1 if either input bit is 1. Using the IPv4 address 128.32.1.14.
In addition to being longer than IPv4 addresses by a factor of 4, IPv6 addresses also have some additional structure. Special prefixes used with IPv6 addresses indicate the scope of an address. The scope of an IPv6 address refers to the portion of the network where it can be used. Important examples of scopes include nodelocal (the address can be used only for communication on the same computer), link-local (used only among nodes on the same network link or IPv6 prefix), or global (Internet-wide). In IPv6, most nodes have more than one address in use, often on the same network interface. Although this is supported in IPv4 as well, it is not nearly as common. The set of addresses required in an IPv6 node, including multicast addresses (see Section 2.5.2), is given in [RFC4291].
In the early 1990s, after the adoption of subnet addressing to ease one form of growing pains, the Internet started facing a serious set of scaling problems. Three particular issues were considered so important as to require immediate attention:
1.By 1994, over half of all class B addresses had already been allocated. It was expected that the class B address space would be exhausted by about 1995.
2.The 32-bit IPv4 address was thought to be inadequate to handle the size of
the Internet anticipated by the early 2000s.
3.The number of entries in the global routing table (one per network number), about 65,000 in 1995, was growing. As more and more class A, B, and C routing entries appeared, routing performance would suffer.
These three issues were attacked by a group in the IETF called ROAD (for ROuting and ADdressing), starting in 1992. They considered problems 1 and 3 to be of immediate concern, and problem 2 as requiring a long-term solution. The short-term solution they proposed was to effectively remove the class breakdown of IP addresses and also promote the ability to aggregate hierarchically assigned IP addresses. These measures would help problems 1 and 3. IPv6 was envisioned to deal with problem 2.