Ip Address Notation - Lucent Technologies PortMaster 4 Configuration Manual

Network Addressing

IP Address Notation

IP addresses are written in dotted decimal notation consisting of four numbers separated
by dots (periods). Each number, written in decimal, represents an 8-bit octet (sometimes
informally referred to as a byte) giving each number a range of 0 through 255, inclusive.
When strung together, the four octets form the 32-bit IP address. Table A-1 shows 32-bit
values expressed as IP addresses.
Table A-1
32-Bit Value
01100100.01100100.01100100.00001010
11000011.00100000.00000100.11001000
The largest possible value of a field in dotted decimal notation is 255, which represents
an octet where all the bits are 1s.
IP Address Classes
IP addresses are generally divided into different classes of addresses based on the
number of hosts and subnetworks required to support the hosts. As described in
RFC 1166, IP addresses are 32-bit quantities divided into five classes. Each class has a
different number of bits allocated to the network and host portions of the address. For
this discussion, consider a network to be a collection of computers (hosts) that have the
same network field values in their IP addresses.
The concept of classes is being made obsolete by classless interdomain routing (CIDR).
Instead of dividing networks by class, CIDR groups them into address ranges. A network
range consists of an IP address prefix and a netmask length. The address prefix specifies
the high-order bits of the IP network address. The netmask length specifies the number
of high-order bits in the prefix that an IP address must match to fall within the range
indicated by the prefix.
For example, 192.168.42.x describes a Class C network with addresses ranging from
192.168.42.0 through 192.168.42.255. CIDR uses 192.168.42.0/24 to describe the same
range of addresses.
RIP-1 is an example of a protocol that uses address classes. RIP-2, OSPF, and BGP-4 are
examples of protocols that do not use address classes.
Class A Addresses
The class A IP address format allocates the highest 8 bits to the network field and sets
the highest-priority bit to 0 (zero). The remaining 24 bits form the host field. Only
126 class A networks can exist (0 is reserved, and 127 is used for loopback networks),
but each class A network can have almost 17 million hosts. No new class A networks
can be assigned at this time.
A-2
IP Address Notation
Dotted Decimal Notation
100.100.100.10
195.32.4.200
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