Classless Interdomain Routing (CIDR)

CLASSLESS INTERDOMAIN ROUTING (CIDR)

Way of describing IP ranges sharing a common bit prefix,we write IP/length, where IP is the first address from the range,and length is the length of the common prefix

Example

We want to describe IP addresses whose binary representation starts with

10011100.00010001.00000100.0010

First IP address from the range:10011100.00010001.00000100.00100000=156.17.4.32 prefix length = 28

Description =156.17.4.32/28

CIDR used mostly for describing single networks 156.17.4.32/28 denotes all the addresses between 156.17.4.32 and 156.17.4.47

·        First address in the network is reserved (network address)

·        Last address is also reserved: broadcast address.

·        Remaining ones can be assigned to computers

CIDR allows creating hierarchie s of networks and subnetworks Note:Top block received directly from IANA.

208.128.0.0/11

Note:

208.130.29.33/32= range consisting of one IPaddress = single computer, not rea lly a network.

We add/xx (called subnet mask) to all IP addresses.

Example:

156.17.4.32/28: denotes whole network 156.17.4.33/28: the first comput er in this network 156.17.4.46/28: the last computer in this network 156.17.4.47/28: broadcast addres s of this network

If you assign address 10.0.0.1 to a network card, it will be interpreted as10.0.0.1/8 Why?

Reason stems from pre-CIDR IP classes

If the first IP bit is 0, assume sub net mask /8(A class network). Example:6.0.0.0/8

If the first IP bits are 10, assume subnet mask/16(B class network). Example:156.17.0.0/16

If the first IP bits are 110, assume subnet mask/24(B classnetwork). Example:200.200.200.0/24

Network 127.0.0.0/8

Interface lo (loopback)

By connecting with any computer from this network (usually with127.0.0.1), you connect with yourself. Application: it is possible to write, test and use network programs without the network.

Reserved ranges of IP addresses

Packet with such addresses should not be passed through routers.Can be used in local networks (same addresses in different networks).

Ranges:

10.0.0.0/8 (one A class network); 172.16.0.0/12 (16 B class networks); 192.168.0.0/16 (256 C class networks)

If computers with private IP addresses want to communicate with the outside world, the connecting router has to perform NetworkAddress Translation(NAT).