Monday, June 20, 2011

Network Classes

  • Class A: Defined by the first 8 bits with a range of 0 - 127.
    First number (8 bits) is defined by Internic i.e. 77.XXX.XXX.XXX
    One class A network can define 16,777,214 hosts.
    Range: 0.0.0.0 - 127.255.255.255

  • Class B: Defined by the first 8 bits with a range from 128 - 191
    First two numbers (16 bits) are defined by Internic i.e. 182.56.XXX.XXX
    One class B network can define 65,534 hosts.
    Range: 128.0.0.0 - 191.255.255.

  • Class C: Defined by the first 8 bits with a range from 192 - 223
    First three numbers (24 bits) are defined by Internic i.e. 220.56.222.XXX
    One class B network can define 254 hosts.
    Range: 192.0.0.0 - 223.255.255.

  • Class D: Defined by the first 8 bits with a range from 224 - 239
    This is reserved for multicast networks (RFC988)
    Range: 224.0.0.0 - 239.255.255.255

  • Class E: Defined by the first 8 bits with a range from 240 - 255
    This is reserved for experimental use.
    Range: 240.0.0.0 - 247.255.255.255


IP Address Class Network and Host Capacities
IP Address Class
Total # Of Bits For Network ID / Host ID
First Octet of IP Address
# Of Network ID Bits Used To Identify Class
Usable # Of Network ID Bits
Number of Possible Network IDs
# Of Host IDs Per Network ID
Class A
8 / 24
0xxx xxxx
1
8-1 = 7
27-2 = 126
224-2 = 16,277,214
Class B
16 / 16
10xx xxxx
2
16-2 = 14
214 = 16,384
216-2 = 65,534
Class C
24 / 8
110x xxxx
3
24-3 = 21
221 = 2,097,152
28-2 = 254

One line of this table so we can see how it works. I'll stick with class B since it's "in the middle". The basic division is into 16 bits for network ID and 16 bits for host ID. However, the first two bits of all class B addresses must be "10”, so that leaves only 14 bits to uniquely identify the network ID. This gives us a total of 214 or 16,384 class B network IDs. For each of these, we have 216 host IDs, less two, for a total of 65,534.

Why less two? For each network ID, two host IDs cannot be used: the host ID with all zeroes and the ID with all ones. These are addresses with "special meanings" as described in the topic that follows. You will also notice that 2 has been subtracted from the number of network IDs for class A. This is because two of the class A network IDs (0 and 127) are reserved. There are actually several other address ranges that are set aside in all three of the classes that I haven't shown here. They are listed in the topic on reserved, private and loopback addresses. (The exclusion of 0 and 127 from class A is probably the best-known address range reservation which is why I am explicit with that one in the table above.)

 

No comments:

Post a Comment

Dear Reader, if you like My Blog content, feel free to comment on our blog posts.