l to a friend, a text representation is used to denote the address information. In IPv4, the 32 bits are broken into four 8-bit (one-byte) fields each separated by a dot. These eight bit fields are presented in decimal numbers, for instance, 198.51.123.234 The network prefix length is variable in IPv4. The size of the prefix is denoted by putting a slash, '1', and the length of the prefix in bits to the end of an address. example, 192.51.100.0/24 shows that the network prefix size is 24 bits. Blocks of addresses are presented in the same way, according to the prefix length. For example, a block of addresses from 198.51.100.0 to 198.51.100.255 is usually written 198.51.100.0/24, or 198.51.100/24 (leaving the zero out at the end). This means that the prefix length is 24 bits, and the host part is the remaining 8 bits out of Deploying IPv6 in 3GPP Networks Table 1.1 IPv4 address classes Address Class Prefix Length Number of Addresses Class A 16,777,216 Class B 65,536 Class C the full 32 bits. When a prefix has more bits, it is considered longer, and a prefix with fewer bits is considered to be shorter. The longer the prefix, the shorter the host part, and hence, fewer addresses are available for the hosts in that network. Originally, the IPv4 address space was divided by the length of the network prefix into different classes. The class A address blocks were blocks of /8 - 16,777,216 addresses, class B address blocks were /16 - 65,536, and class C blocks of /24 - 256 addresses. Table 1.1 shows the address classes and their lengths. In the 1990s, the Internet moved to classless addressing - Classless Inter-Domain Routing (CIDR) [36, 37] - and the network prefix length is no longer dependent on the address class; the length can be different from the original address classes. The addresses used for normal unicast communication between hosts are called unicast addresses. Quite obviously, the main part of the allocated address space is dedicated unicast addresses. These addresses are sometimes also called public addresse