Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

Ethernet and TCP/IP networks 339
Finally, the source and destination addresses are the 32-bit IP addresses of the origin
and the destination hosts of the datagram.
IPv4 addressing
The ultimate responsibility for the issuing of IP addresses is vested in the Internet
Assigned Numbers Authority (IANA). This responsibility is, in turn, delegated to the three
Regional Internet Registries (RIRs) viz. APNIC (Asia-Pacific Network=Information
Centre), ARIN (American Registry for Internet Numbers), and RIPE NCC (Reseau IP
Europeans). RIRs allocate blocks of IP addresses to Internet service providers (ISPs)
under their jurisdiction, for subsequent issuing to users or sub-ISPs.
The IPv4 address consists of 32 bits, e.g. 11000000011001000110010000000001. Since
this number is fine for computers but a little difficult for human beings, it is divided into
four octets w, x, y and z. Each octet is converted to its decimal equivalent. The result of the
conversion is written in the format 192.100.100.1. This is known as the ‘dotted decimal’ or
‘dotted quad’ notation. As mentioned earlier, one part of the IP address is known as the
network ID or ‘NetID’ while the rest is known as the ‘HostID’.
Originally, IP addresses were allocated in so-called address classes. Although the system
proved to be problematic, and IP addresses are currently issued ‘classless’, the legacy of IP
address classes remains and has to be understood. To provide for flexibility in assigning
addresses to networks, the interpretation of the address field was coded to specify either a
small number of networks with a large number of hosts (class A), or a moderate number of
networks with a moderate number of hosts (class B), or a large number of networks with
a small number of hosts (class C). There was also provision for extended addressing
modes: class D was intended for multicasting whilst E was reserved for future use.
Figure 12.15
Address structure for IPv4
For class A, the first bit is fixed at 0. The values for ‘w’ can therefore only vary between
0 and 127 10
. 0 is not allowed and 127 is a reserved number used for testing. This allows for
126 class A NetIDs. The number of HostIDs is determined by octets ‘x’, ‘y’ and ‘z’. From
these 24 bits, 2 24 = 16 777 218 combinations are available. All zeros and all ones are not
permissible, which leaves 16 777 216 usable combinations.
For class B, the first two bits are fixed at 10. The binary values for ‘w’ can therefore
only vary between 128 10
and 191 10
. The number of NetIDs is determined by octets ‘w’ and
‘x’. The first 2 bits are used to indicate class B and hence cannot be used. This leaves