IP Addressing
IP address descriptions are found in RFC 1166, Internet Numbers. The
Internet Network Information Center (InterNIC) maintains and distributes
the RFC documents. The InterNIC also assigns IP addresses and network
numbers to Internet Service Providers (ISPs), who in turn provide to their
customers a range of addresses appropriate to the number of host devices
on their network.
The section that follows describe the various types of IP addresses, how
addresses are given, and routing issues related to IP.
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 for the 32-bit IP address.
IP Address Notation
32-Bit Value
Dotted Decimal Notation
01100100.01100100.01100100.00001010
100.100.100.10
11000011.00100000.00000100.11001000
195.32.4.200
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 supporting 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 example, 192.168.45.x describe a Class 'C' network with addresses
ranging from 192.168.45.0 through 192.168.168.255.
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' network can be
assigned at this time.
Class 'B' Addresses
The class 'B' IP address format allocates the highest 16 bits to the
network field and sets the two highest-order bits to 1 and 0, providing a
range from 128 through 191, inclusive. The remaining 16 bits providing a
range from 128 to 191, inclusive. The remaining 16 bits form the host
field. More than 16,000 class 'B' networks can exist, and each class 'B'
network can have up to 65,534 hosts.
Class 'C' Addresses
The class 'C' IP address format allocates the highest 24 bits to the
network field and sets the three highest-order bits to 1,1, and 0,
providing a range from 192 through 223, inclusive. The remaining 8 bits
form the host field. More than two million class 'C' networks can exist,
and each class 'C' network can have up to 254 hosts.
Class 'D' Addresses
The class 'D' IP address format was designed for multicast groups, as
discussed in RFC 988. In class 'D' addresses, the 4 highest-order bits
are set 1, 1, 1, and 0, providing a range from 224 through 239, inclusive.
Class 'D' addresses is currently used primarily for the multicast backbone
(MBONE) of the Internet. Many routers and firewalls do not support MBONE
or multicast and therefore ignore class 'D' addresses.
Class 'E' Addresses
The Class 'E' IP address is reserved for future use. In Class 'E'
addresses, the 4 highest-order bits are set to 1, 1, 1, and 1. Routers
and Firewalls currently ignore class 'E' IP addresses.
Reserved IP Addresses
Some IP addresses are reserved for special uses and cannot be used for
host addresses. The following table lists ranges of IP addresses and shows
which addresses are reserved, which are available to be assigned and which
are for broadcast.
Class
IP Address
Status
A
0.0.0.0
Reserved
1.0.0.0 through 126.0.0.0
Available
127.0.0.0
Loopback networks on the local host
B
128.0.0.0
Reserved
128.1.0.0 through 191.254.255.255
Available
191.255.0.0
Reserved
C
192.0.0.0
Reserved
192.0.1.0 through 223.255.254.255
Available
223.255.255.0
Reserved
D
224.0.0.0 through 239.255.255.255
Multicast group addresses
E
240.0.0.0 through 255.255.255.254
Reserved
255.255.255.255
Broadcast
Private IP Networks
RFC 1918 reserved three IP network addresses for private networks. The
addresses 10.0.0.0/8, 192.168.0.0/16 and 172.16.0.0/20 can be used for
anyone for setting up their own internal IP networks.
Johann van Duyn <[EMAIL PROTECTED]>
Sent by: [EMAIL PROTECTED]
02/23/00 04:36 AM
To: [EMAIL PROTECTED]
cc:
Subject: Private IP address ranges?
Hi...
I am planning on using NAT on my firewall, and need to know what IP
address ranges are safe to use on my internal network, so as not to have
addresses on my internal network that are the same as those of actual
sites on the Internet. Could anyone please either tell me what the correct
ranges would be for class A, B and C, or point me to a resource where I
could find that info?
Thanks
Johann van Duyn
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