As mentioned earlier, reverse address resolution protocol (RARP) is used for diskless computers to determine their IP address using the network. The RARP message format is very similar to the ARP format. When the booting computer sends the broadcast ARP request, it places its own hardware address in both the sending and receiving fields in the encapsulated ARP data packet. The RARP server will fill in the correct sending and receiving IP addresses in its response to the message. This way the booting computer will know its IP address when it gets the message from the RARP server.
Network Addressing
IP addresses are broken into 4 octets (IPv4) separated by dots called dotted decimal notation. An octet is
a byte consisting of 8 bits. The IPv4 addresses are in the following form:
192.168.10.1
There are two parts of an IP address:
- l Network ID
- l Host ID
The various classes of networks specify additional or fewer octets to designate the network ID versus the
host ID. When a network is set up, a netmask is also specified. The netmask determines the class of the network
as shown below, except for CIDR. When the netmask is setup, it specifies some number of most
significant bits with a 1's value and the rest have values of 0. The most significant part of the netmask
with bits set to 1's specifies the network address, and the lower part of the address will specify the host
address. When setting addresses on a network, remember there can be no host address of 0 (no host
address bits set), and there can be no host address with all bits set.
Class A-E networks
The addressing scheme for class A through E networks is shown below. Note: We use the 'x' character
here to denote don't care situations which includes all possible numbers at the location. It is many times
used to denote networks.
Network Type Address Range Normal Netmask Comments
Class A 001.x.x.x to 126.x.x.x 255.0.0.0 For very large networks
Class B 128.1.x.x to 191.254.x.x 255.255.0.0 For medium size networks
Class C 192.0.1.x to 223.255.254.x 255.255.255.0 For small networks
Class D 224.x.x.x to 239.255.255.255 Used to support multicasting
Class E 240.x.x.x to 247.255.255.255
RFCs 1518 and 1519 define a system called Classless Inter-Domain Routing (CIDR) which is used to
allocate IP addresses more efficiently. This may be used with subnet masks to establish networks rather
than the class system shown above. A class C subnet may be 8 bits but using CIDR, it may be 12 bits.
There are some network addresses reserved for private use by the Internet Assigned Numbers Authority
(IANA) which can be hidden behind a computer which uses IP masquerading to connect the private
network to the internet. There are three sets of addresses reserved. These address are shown below:
