Classless addressing

Classless addressing

Classful addressing has created many problems
Many ISPs and service users need more addresses
Idea is to have variable-length blocks that belong to no class
Three restrictions on classless address blocks;
The addresses in a block must be contiguous, one after another
The number of addresses in a block must be a power of 2
The first address must be evenly divisible by the number of addresses


Mask and Address Blocks

In IPv4 addressing, a block of addresses can be defined as x.y.z.t /n in which x.y.z.t defines one of the addresses and the /n defines the mask.
The first address in the block can be found by setting the rightmost 32 − n bits to 0s
The last address in the block can be found by setting the rightmost 32 − n bits to 1s
The number of addresses in the block can be found by using the formula 232−n

Example: 205.16.37.39/28
The binary representation is 1100110 00010000 00100101 00100111
If we set 32 − 28 rightmost bits to 0, we get 11001101 00010000 00100101 00100000
 205.16.37.32 (First address)
If we set 32 − 28 rightmost bits to 1, we get 11001101 00010000 00100101 00101111
 205.16.37.47 (Last address)
The value of n is 28, which means that number of addresses is 232−28 or 16


Network Address

The first address in a block is normally not assigned to any device; it is used as the network address that represents the organization to the rest of the world

Two-Level Hierarchy: No Subnetting

Each address in the block can be considered as a two-level hierarchical structure: the leftmost n bits (prefix) define the network; the rightmost 32 − n bits define the host


Address Allocation and Distribution: Example

The first group has 64 customers; each needs 256 addresses.
The second group has 128 customers; each needs 128 addresses.
The third group has 128 customers; each needs 64 addresses.


Network Address Translation: NAT

NAT enables a user to have a large set of addresses internally and one address, or a small set of addresses, externally.


Addresses Translation
Address translation for source address of outgoing packet and for destination address of incoming packet

Translation Table

Using (1) one IP address, (2) a pool of IP address, and (3) both IP addresses and port numbers

Five-Column Translation Table

IPv6 Addresses

Despite all short-term solutions, such as classless addressing, DHCP (Dynamic Host Configuration Protocol), and NAT, still address-hungry
An IPv6 address is 128 bits long
Hexadecimal colon notation:


IPv6 Address Space

IPv6 Addresses

Unicast addresses: define a single computer
Two types: geographically based and provider-based
Prefixes for provider-based unicast address
Type id (3 bits), Registry id (5 bits)

IPv6 Addresses

Anycast addresses: define a group of nodes
Unlike multicast, a packet is delivered to only one of the members of the anycast group, the nearest