acket handler IP-IDU IP-SDU Fig. 3.77 Transport over ISDN packet access In this scenario we assume that ISDN connections between hosts and the IP packet handler are set up on demand (i.e. as CN connections over B-channels). The user of the host must therefore have access to the ISDN terminal process controller, to create a CN connection (a B-channel) between the CNU in the host and the CNU in the packet handler. The LAPB element in the packet handler stores the relation between the B-channel and the ISDN address of the hosts, and the IP-UDT stores the relation between the ISDN address and the IP address in its AT. Thus, there is a two-stage translation to perform for this type of route. A CN connection and LAPB impose no restriction on the size of IP-SDUs, SO there is no fragmentation to be done over these routes (note, however, that the reliability of data transfer decreases with the size of SDUs sent over LAPB). IP addresses must be carried by IP. The number of bits that are available in the protocol determines the maximum number of hosts that can be served by an IP net- work. The original IP addressing system (IPv4, see the standard RFC 791 by the Internet Society, ISOC) theoretically has room for 4 294 967 296 hosts (32-bits addresses). In reality, however, only around 200 million are usable, due to the way addresses are managed. The developers of IPv4 did not foresee the exceptional increase in the number of connected hosts, which, in the Internet, are expected to exceed the capacity of IPv4 soon. This was the determining factor that recently forced modification of the IP to (from IPv4 to IPv6). Among other things, the address size was extended from 32 to 128 bits. IPv6 is described by ISOC-standard RFC 1883. Addresses are the means for routing in networks, since a destination address determines which path user data will take in the network. Addressing principles, sometimes called numbering plans, are therefore an important part of any network standard, see e.g. ITU-T E. 163, E. 164, 1.330 for PSTN, ISDN and