Addressing Ip - Sagem ADR 155C Installation And User Manual

ANNEX A - CONSTRUCTING AN IP NETWORK ADDRESSING PLAN
A.2 - Addressing IP
To route packets in a node, IP uses an addressing mechanism: depending on the destination
address contained in the packet, it determines by polling its routing table the next router to which
it should send the packet, and determines the physical interface over which it should transmit the
packet. (The terms "router" and "equipment" will be used interchangeably in the following),
There are two, non-exclusive, ways of updating IP routing tables:
• use of static routes entered by operator,
• use of a routing demon (such as RIP, RIP2, OSPF) which takes charge of dynamic
management of routing tables across the network.
The IP addresses are coded over 4 bytes and are usually represented in decimal form (e.g.:
135.11.33.115). An IP address is divided into 3 fields:
• the "NET" field, on the left,
• the "subnet" field, in the center,
• the "host" field, on the right.
The length of the first field depends on the class to which the address belongs. The length of the
"subnet" and "host" fields is defined by a mask associated with the address. There are three
address classes:
• class A: the "NET" field is coded over the first byte; the three other bytes are reserved
for the
126.255.255.254,
• class B: the "NET" field is coded over the first two bytes; the two other bytes are
reserved for the
128.0.0.0 to 191.255.255.254,
• class C: the "NET" field is coded over the first three bytes; the last byte is reserved for
the "SUBNET" and "HOST" fields ; class C addresses vary from 192.0.0.0 to
223.255.255.254 ; the maximum number of equipment pieces that can belong to the
same class C IP network (having the same "NET") is 254.
Note: Addresses starting with 127 and addresses including "all-1" or "all-0" in the "SUBNET"
and "HOST" fields are not permitted.
Note: The IP standard authorizes that the "SUBNET" and "HOST" fields can be coded over a
number of bits that does not correspond to an byte border; usually this is not
implemented for reasons of address legibility and since the majority of RIP routing
demon implementations do not support this feature.
The "SUBNET" field is defined by a mask which is written in the form: 255.255.255.0,
255.255.0.0, 255.0.0.0, etc : the number of zero bits from the left of the mask defines the length
of the "HOST" field ; e.g., for the address 40.2.2.2 associated with mask 255.255.255.0, the
"NET" field takes the value 40, the "SUBNET" field the value 2.2 and the "HOST" field the
value 2.
Installation and User Guide - N56717020101
Page AA-2
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"SUBNET" and "HOST" fields ; class A addresses vary from 1.0.0.0 to
"SUBNET" and "HOST" fields ; class B addresses vary from