CTI Janus Programmable Automation Controller IOG
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APPENDIX B: IP ADDRESS INFORMATION
IP Address Nomenclature
IP Address
Every host interface on a TCP/IP network is identified by a unique IP Address. This address is used to uniquely
identify the host device, such as a workstation or communications module, and the network to which the host
belongs.
Each IPV4 Address consists of 32 bits, divided into four 8 bit entities called
octets
. An IP Address is expressed in
dotted notation,
with each octet expressed as its decimal equivalent. See the example below.
Notation
Octet 1
Octet 2
Octet 3
Octet 4
Binary
11000000
11011111
10110001
00000001
Decimal
192
223
177
1
Although an IP Address is a single value, it contains two types of information: the
Network ID
and the
Host ID
. The
Network ID identifies the IP network to which the host belongs. The Host ID identifies a specific IP host on that IP
network. All IP hosts on a particular local area network must have the same network ID. Each IP host on a
particular local area network must use a unique Host ID.
Address Classes
The Internet community originally defined network classes to accommodate networks of varying sizes. The
network class can be discerned from the first octet of its IP Address.
The following table summarizes the relationship between the first octet of a given address and its Network ID and
Host ID fields. It also identifies the total number of Network IDs and Host IDs for each address class that
participates in the Internet addressing scheme.
Class
First Octet
Value*
Network ID
Host ID
Number of
networks
Number of hosts
per net
A
1-126
First Octet
Last 3 Octets
126
16,777,214
B
128-191
First 2 Octets
Last 2 Octets
16,384
65,534
C
192-223
First 3 Octets
Last Octet
2,097,151
254
* Address 127 is reserved for loopback testing and inter-process communication on the local computer; it is not a
valid network address. Addresses 224
–
239 are used for Class D (IP multicast).
