Nortel Optical Multiservice Edge 6130, Руководство по планированию, Страница: 280 / 292

10-94 Appendix A: Data communications planning
Optical Multiservice Edge 6130 NT6Q92MA Rel 1.0 Iss 1 Standard September 2006
It is important to note that the original distinction of addresses between classes
A, B, and C did not allow the network designer to vary the length of the
subnetwork mask once an IP address was assigned. In fact, classes A, B, and
C defined a rigid scheme for assigning the number of bits which identify the
networks, and those which identify the host ID, regardless of the network
designer’s needs. For its intrinsic limits, the notation for classes A, B, and C is
no longer used.
The next section of this information shows the bit significance of the dotted
decimal notation.
Dotted decimal notation for IP addresses
An IP address is 32 bits long. To aid memory of this string of digits, the address
is split into 4 groups of 8 bits each. These are represented by a decimal number
between 0 and 255 for ease of display. The decimal number has no other
significance.
The decimal number represented by the eight bits can be determined by
looking up the decimal value of each bit and then simply adding them up:
• 00000001 is represented by 1
• 00000010 is represented by 2
• 00000100 is represented by 4
• 00001000 is represented by 8
• 00010000 is represented by 16
• 00100000 is represented by 32
• 01000000 is represented by 64
• 10000000 is represented by 128
Example: 10010001
• 10000000 is represented by 128
• 00010000 is represented by 16
• 00000001 is represented by 1
• Total 145
An IP address contains a 32-bit address field and a 32-bit subnet mask. The
mask defines which part of the address is a network address and which is a
device address. The mask thus allows a router to decide whether the address of
the packet is destined for one of the subnets to which it is connected. For
example:
An address of 47.9.64.172 has a mask of 255.255.240.0; the mask is
AND’ed with the address: