Classless addressing. Types of IPv4 Addresses

Introduction
Prior to 1981, IP addresses used only the first 8 bits to specify the network portion of the address
In 1981, RFC 791 modified the IPv4 32-bit address to allow for three different classes
Class A addresses used 8 bits for the network portion of the address,
Class B used 16 bits,
Class C used 24 bits.
This format became known as classful IP addressing.
IP address space was depleting rapidly
the Internet Engineering Task Force (IETF) introduced Classless Inter-Domain Routing (CIDR)
CIDR uses Variable Length Subnet Masking (VLSM) to help conserve address space.
-VLSM is simply subnetting a subnet

Classful and Classless IP Addressing
Classful IP addressing
When the ARPANET was commissioned in 1969, no one anticipated that the Internet would explode.
1989, ARPANET transformed into what we now call the Internet.
As of January 2007, there are over 433 million hosts on internet
Initiatives to conserve IPv4 address space include:
-VLSM & CIDR notation (1993, RFC 1519)
-Network Address Translation (1994, RFC 1631)
-Private Addressing (1996, RFC 1918)

Classful and Classless IP Addressing
The IPv4 Classful Addressing Structure (RFC 790)
An IP address has 2 parts:
-The network portion
Found on the left side of an IP address
-The host portion
Found on the right side of an IP address

Classful and Classless IP Addressing
As shown in the figure, class A networks used the first octet for network assignment, which translated to a 255.0.0.0 classful subnet mask.
Because only 7 bits were left in the first octet (remember, the first bit is always 0), this made 2 to the 7th power or 128 networks.
With 24 bits in the host portion, each class A address had the potential for over 16 million individual host addresses.

Classful and Classless IP Addressing
With 24 bits in the host portion, each class A address had the potential for over 16 million individual host addresses.
What was one organization going to do with 16 million addresses?
Now you can understand the tremendous waste of address space that occurred in the beginning days of the Internet, when companies received class A addresses.
Some companies and governmental organizations still have class A addresses.
General Electric owns 3.0.0.0/8,
Apple Computer owns 17.0.0.0/8,
U.S. Postal Service owns 56.0.0.0/8.

Classful and Classless IP Addressing
Class B: RFC 790 specified the first two octets as network.
With the first two bits already established as 1 and 0, 14 bits remained in the first two octets for assigning networks, which resulted in 16,384 class B network addresses.
Because each class B network address contained 16 bits in the host portion, it controlled 65,534 addresses. (Remember, 2 addresses were reserved for the network and broadcast addresses.)

Classful and Classless IP Addressing
class C: RFC 790 specified the first three octets as network.
With the first three bits established as 1 and 1 and 0, 21 bits remained for assigning networks for over 2 million class C networks.
But, each class C network only had 8 bits in the host portion, or 254 possible host addresses.

Classful and Classless IP Addressing
Classless Inter-domain Routing (CIDR – RFC 1517)
Requires subnet mask to be included in routing update because address class is meaningless
The network portion of the address is determined by the network subnet mask, also known as the network prefix, or prefix length (/8, /19, etc.).
The network address is no longer determined by the class of the address
Blocks of IP addresses could be assigned to a network based on the requirements of the customer, ranging from a few hosts to hundreds or thousands of hosts.
CIDR – use arbitrary prefix length of Network ID
E.g. 205.100.0.0/22 means that network ID length is 22 bits, i.e. netmask is 255.255.252.0

Lecture 7: 9-18-01
Classless Inter-Domain Routing
Do not use classes to determine network ID
Assign any range of addresses to network
Use common part of address as network number
E.g., addresses 192.4.16 - 192.4.31 have the first 20 bits in common. Thus, we use these 20 bits as the network number
netmask is /20, /xx is valid for almost any xx
Enables more efficient usage of address space (and router tables)

Lecture 7: 9-18-01
CIDR Example
Network provide is allocated 8 class C chunks, 201.10.0.0 to 201.10.7.255
Allocation uses 3 bits of class C space
Remaining 21 bits are network number, written as 201.10.0.0/21
Replaces 8 class C routing entries with 1 combined entry
Routing protocols carry prefix with destination network address
Longest prefix match for forwarding

Lecture 7: 9-18-01
CIDR Illustration
Provider is given 201.10.0.0/21
201.10.0.0/22
201.10.4.0/24
201.10.5.0/24
201.10.6.0/23
Provider

Classless Inter-Domain Routing (CIDR)
The general form of CIDR notation is: ddd.ddd.ddd.ddd/m
ddd is the decimal value for an octet of the address
m is the number of one bits in the mask
Consider the mask needed for a network with 28 bits of prefix:
It has 28-bits of 1s followed by 4-bits of 0s
In dotted decimal, the mask is: 255.255.255.240
In CIDR notation,
the mask is written:
128.211.0.16/28
which specifies
a mask with 28 bits
of prefix and 4 bits
of suffix.

IPv4 Address Structure
Network, Host, and Broadcast Addresses
Within each network are three types of IP addresses:
Network address
Host addresses
Broadcast address

IPv4 Address Structure
Network, Host, and Broadcast Addresses
Types of Addresses in Network 192.168.10.0/24
Network Address - host portion is all 0s (.00000000)
First Host address - host portion is all 0s and ends with a 1 (.00000001)
Last Host address - host portion is all 1s and ends with a 0 (.11111110)
Broadcast Address - host portion is all 1s (.11111111)

Network, Host, and Broadcast Addresses (cont.)

Network, Host, and Broadcast Addresses (cont.)

Network, Host, and Broadcast Addresses (cont.)

Network, Host, and Broadcast Addresses (cont.)