(T) Initially, Internet Protocol version 4 (IPv4) addresses were defined in IETF RFC 791 as a two-part object combining: a “network identifier” and a “host identifier”. The network identifiers are assigned by IANA. The host identifiers are assigned by the network manager. There are five classes of addresses: A (7 bits for the network number/24 bits for the host number) B (14 for the net/16 for the hosts) C (21 for the net/8 for the hosts), D (for multicast) and E (experimental). Each address is normally represented as four decimal numbers separated by dots such as 128.88.12.7. Class A/B/C are called classful addressing.

Preserving Internet Addresses

One of the first problems encountered with IPv4 addressing was the market demand for class B. To cope with that challenge, Internet routing with classful addressing was replaced in 1993 by Classless Inter-Domain Routing (CIDR). CIDR introduces an extension called supernetting or network prefix to the classful address: 128.88.12.7/8.

Classless Addressing and Supernetting:
Internet routers use this network prefix or classless addressing instead of the full IP address to route IP packets on the Internet. The supernet can be seen as the country or area code for the telephone network. Supernets collapse a number of contiguous addresses into one by providing the pair: Network address/Network Prefix Length, where the network address is the first address in the contiguous block and the network prefix length allows interfering the full network address, through a mask comparison, in the supernet.

CIDR:
With CIDR, a single route advertisement can, therefore, cover a block or old-style addresses and so addresses can be assigned hierarchically. This means that large blocks are delegated to large service providers, which, then, can break up their allocation, keep some, and delegate smaller blocks to smaller providers.

Internet routing with classless addressing is performed with the Border Gateway Protocol (BGP) version 4. BGP is a “path vector” protocol.  BGP handles routing communication-based on configured policies between Autonomous System (AS), which defines “a set of networks under the same management organization”. BGP routers exchange network reachability information which lists the complete path for each and every possible AS destination on the Internet between service providers.

Preserving and Managing Addresses on the LAN

Like service providers on the Internet, enterprise networks have adopted for a long time various protocols and tools to better manage the growth of their IP addresses in particular with subnets, DHCP and NAT.

Subnetting:
Subnetting for the LAN is the equivalent of supernetting for the Internet. The subnet address format is:
Network number- Subnet-Host number
The subnet can have any length and is specified by a “mask” determined by a “comparison-under-mask” operation. Enterprise networks have designed subnets to subdivide their IP networks into unequal pieces, each having their own subnet mask. The network administrator “right sizes” the addressing of each subnet with variable-length subnet mask (VLSM). VLSM like CIDR makes efficient use of an organization’s assigned IP address space. It reduces as well the amount of routing information by leveraging the subnet to route packets to their final destination.

DHCP:
When an IP address has been given to a host, it can be found using the Address Resolution Protocol (ARP) that map the IP address to the equipment hardware Ethernet MAC address. To simplify adding, changing and moving IP addresses in an enterprise network, Dynamic Host Configuration Protocol (DHCP) was created. IP addresses can be assigned for a limited time or until the end station relinquishes it. DHCP supplies IP host address, subnet mask, and local gateway information in response to end-system broadcast requests.

NAT:
Network Address Translation (NAT) was designed for enterprise networks that are not fully connected to the Internet and accept having all their outgoing traffic “rewritten” by an address translation gateway. With NAT, the “partially connected” network uses “private addresses”, that is addresses that are not routable in the Internet and are not “public”. This enables NAT to be used for securing the entrance to private networks.

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