System components
2.7 IPv4 networks
2.7.1 IP addresses
Every network interface on the Internet needs to have a unique number which is called its address. IP addresses are organized hierarchically so that they can be searched for by router networks. Without such a structure, it would be impossible to find a host unless it were part of the same cable segment. At present the Internet protocol is at version 4 and this address consists of four bytes, or 32 bits.
In the future this will be extended, in a new version of the Internet protocol IPv6, to allow more IP addresses since we are rapidly using up the available addresses.
The addresses will also be structured differently. The form of an IP address in IPv4 is
aaa.bbb.ccc.mmm
Some IP addresses represent networks, whereas others represent individual inter-faces on hosts and routers. Normally an IP address represents a host attached to a network.
In every IPv4 address there are 32 bits. One uses these bits in different ways:
one could imagine using all 32 bits for host addresses and keep every host on the same enormous cable, without any routers (this would be physically impossible in practice), or we could use all 32 bits for network addresses and have only one host per network (i.e. a router for every host). Both these extremes are silly; we are trying to save resources by sharing a cable between convenient groups of hosts, but shield other hosts from irrelevant traffic. What we want instead is to group hosts into clusters so as to restrict traffic to localized areas.
Networks were grouped historically into three classes called class A, class B and class C networks, in order to simplify traffic routing (see chapter 10). Class D and E networks are also now defined, but these are not used for regular traffic. This rigid distinction between different types of network addresses has proved to be a costly mistake for the IPv4 protocol. Amongst other things, it means that only about two percent of the actual number of IP addresses can actually be used with this scheme. So-called classless addresses (CIDR) were introduced in the 1990s to patch the problem of the classed addressing, but not all deployed devices and protocol versions were able to understand the new classless addresses, so classed addressing will survive in books and legacy networks for some time.
The difference between class A, B and C networks lies in which bits of the IP addresses refer to the network itself and which bits refer to actual hosts within a network. Note that the details in these sections are subject to rapid change, so readers should check the latest details on the web.
Class A legacy networks
IP addresses from 1.0.0.0 to 127.255.255.255 are class A networks. Originally only 11.0.0.0 to 126.255.255.255 were used, but this is likely to change as the need for IPv4 address space becomes more desperate. In a class A network, the first byte is a network part and the last three bytes are the host address (see figure 2.8). This allows 126 possible networks (since network 127 is reserved for the loopback service). The number of hosts per class A network is 2563 minus reserved host addresses on the network. Since this is a ludicrously large number, none of the owners of class A networks are able to use all of their host addresses.
Class A networks are no longer issued (as class A networks), they are all assigned, and all the free addresses are now having to be reclaimed using CIDR. Class A networks were intended for very large organizations (the U.S. government, Hewlett Packard, IBM) and are only practical with the use of a netmask which divides up the large network into manageable subnets. The default subnet mask is 255.0.0.0.
0 8 16 24 32
CLASS B
CLASS D 1
1 1 0
0 1 1 1 1 1 1 0
0 Network
Network
Network
Experimental – reserved for future Multicast addresses
Host Host Host
0
1 CLASS A
CLASS C
CLASS E
Figure 2.8: Bit view of the 32 bit IPv4 addresses.
Class B legacy networks
IP addresses from 128.0.0.0 to 191.255.0.0 are class B networks. There are 16,384 such networks. The first two bytes are the network part and the last two bytes are the host part. This gives a maximum of 2562 minus reserved host addresses, or 65,534 hosts per network. Class B networks are typically given to large institutions such as universities and Internet providers, or to institutions such as Sun Microsystems, Microsoft and Novell. All the class B addresses have now been allocated to their parent organizations, but many of these lease out these addresses to third parties. The default subnet mask is 255.255.0.0.
Class C legacy networks
IP addresses from 192.0.0.0 to 223.255.255.0 are class C networks. There are 2,097,152 such networks. Here the first three bytes are network addresses and the last byte is the host part. This gives a maximum of 254 hosts per network. The default subnet mask is 255.255.255.0. Class C networks are the most numerous and there are still a few left to be allocated, though they are disappearing with alarming rapidity.
Class D (multicast) addresses
Multicast networks form what is called the MBONE, or multicast backbone. These include addresses from 224.0.0.0 to 239.255.255.0. These addresses are not normally used for sending data to individual hosts, but rather for routing data to multiple destinations. Multicast is like a restricted broadcast. Hosts can ‘tune in’
to multicast channels by subscribing to MBONE services.
Class E (Experimental) addresses
Addresses 240.0.0.0 to 255.255.255.255 are unused and are considered exper-imental, though this may change as IPv4 addresses are depleted.
Other addresses
Some IP addresses are reserved for a special purpose. They do not necessarily refer to hosts or networks.
0.0.0.0 Default route
0.*.*.* Not used
127.0.0.1 Loopback address 127.*.*.* Loopback network
*.*.*.0 Network addresses (or old broadcast)
*.*.*.255 Broadcast addresses
*.*.*.1 Router or gateway (conventionally) 224.*.*.* Multicast addresses
RFC 1918 defines private addresses that are not routed 10.0.0.0 - 10.255.255.255 (10/8 prefix) 172.16.0.0 - 172.31.255.255 (172.16/12 prefix) 192.168.0.0 - 192.168.255.255 (192.168/16 prefix) and as of July 2001
169.254.0.0 - 169.254.255.255 (192.254/16 prefix) The network
192.0.2.0 - 192.0.2.255
is reserved by RFC 1166 to be the domain example.org for testing and example (as in this book).
Note that older networks used the network address itself for broadcasting.
This practice has largely been abandoned however. The default route is a default destination for outgoing packets on a subnet and is usually made equal to the router address.
The loopback address is an address which every host uses to refer to itself internally. It points straight back to the host. It is a kind of internal pseudo-address which allows programs to use network protocols to pseudo-address local services without anything being transmitted on an actual network.
The zeroth address of any network is reserved to mean the network itself, and the 255th (or on older networks sometimes the zeroth) is used for the broadcast address. Some Internet addresses are reserved for a special purpose.
These include network addresses (usually xxx.yyy.zzz.0), broadcast addresses (usually xxx.yyy.zzz.255, but in older networks it was xxx.yyy.zzz.0) and multicast addresses (usually 224.xxx.yyy.zzz).