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Extended Network Prefix. ❑ Internet routers use only the network-prefix of the destination address to route traffic to a subnetted environment.
Typology: Schemes and Mind Maps
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Department Computer Science
Telecommunication Program
Outline
Internet Address Structure
o Classfull Addresses
o Classeless Addresses
o Subnetting and Supernetting
DHCP and ARP
Network Address Translation
Every device connected to the public Internet is
assigned a unique IP address.
An IP address can sometimes be used to show the user's
general location.
IP addresses can be assigned by an ISP statically
(Static IP Address) or dynamically (Dynamic IP
Address)
Class A Networks
Class B Networks
Class C Networks
IP Addresses
IP Address dotted decimal notation
o It divides the 32-bit IP address into 4 byte fields and
specifies each byte independently as a decimal
number with the fields separated by dots
10 010001 00001010 00100010 00000011
145 145 145 145
145.10.34.
Unforeseen Limitation of Classfull
Addressing
Addresses were allocated to organizations based their
requests rather than actual need
The decision to standardize on a 32-bit address space
did not foresee a network of things
Classes were easy to understand and implement but
did not foster efficient allocation
Subnet Addresses
Subnetting is introduced mainly to address
depletion and routing table inflation.
Three-Level Hierarchy
o The number of subnets must be a power of 2
The subnet structure of a network is
never visible outside the local network
o This limits considerably the expansion of the
routing table
10 NetID SubnetID HostID
Sub-netting
Subnet 1
128.143.1.h
Subnet 2
128.143.2.h
Subnet 3
128.143.3.h
Extended Network Prefix
Internet routers use only the network-prefix
of the destination address to route traffic to
a subnetted environment
o Routers within the subnetted environment use
the extended network prefix to route traffic
NetID SubnetID HostID
Extended Network Prefix
Subnetting – Prefix Advertisement
Internet
Subnet ID
Private Network
Default Mask
Subnet Mask
Default Mask and Subnet Mask
Subnetting - Example
A company is granted the site address
201.70.64.0: Class C address.
The number of 1s in the default mask is 24
The company requires six subnets
How can such a network be designed?
Subnetting Solution
The number 6 is not a power of 2.
o The next power of 2 is 8 (
)
o 3 more bits are needed for the subnet
mask
The total number of 1s in the subnet mask is
27
CIDR was designed to address the ROADs
problem
No concept of address classes
Prefixes are not restricted to /8, /16 and /
o Prefixes could be any length from 1 to 32
As a result, CIDR supports the deployment of
arbitrarily sized networks rather than the standard
8 - bit, 16-bit or 24-bit networks numbers
o Regardless of the class of the IP address, a network with
20 bits of network-number and 12 bits of host number is
advertised with a 20-bit prefix length
Classfull Subnetting and Address
Allocation
1
4
2
7
8
12
13
14
15
0
6
9
10
11
3
5
ISP owns the address block 200.25.0.0/16 and wants to
allocate the 20.25.16.0/20 address block
In a classful environment, it can only be cut into 16
equal-size segments
Classless Subnetting and Address
Allocation
A
Address slices do not have to be of equal size
Address block 200.25.16.0/
C
D
B
CIDR Address Allocation
Divide reserved address block
200.25.28.0/22 into 2 equal slices
o Each block represents one-half of the
address space – 512 IP addresses
Controlling Routing Table Growth
CIDR requires that the Internet be divided into
addressing domains
o Within a domain, detailed information is available
about all networks that reside in the domain
o Outside of an addressing domain, only the common
network prefix is advertised
This allows single routing table entry to
specify a route to many individual network
addresses
CIDR – Controlling Routing Table
Growth
Internet
Organization A
Organization B Organization C Organization D
CIDR Forwarding Algorithm
All routers must implement a consistent forwarding
algorithm based on the "longest match" algorithm.
o A route with a longer extended-network-prefix describes a
smaller set of destinations than the same route with a
shorter extended-network-prefix.
o A route with a longer extended-network-prefix is said to
be "more specific" while a route with a shorter extended-
network-prefix is said to be "less specific."
Routers must use the route with the longest
matching network-prefix (most specific matching
route) when forwarding traffic.
Routing in Classless Environments
Organization A changes its network provider to ISP #
Best scenario – obtain a block of ISP #2’s address space
and renumber
o In practice, renumbering is hard and costly
Internet
Internet Service
Provider #
Internet Service
Provider #
Organization A
Routing in Classless Environments
A more practical solution:
o Retain ownership of its original address space (ISP #1)
o Have ISP #2 advertise an “exception”, more specifically
Internet
Internet Service
Provider #
Internet Service
Provider #
Organization A
CIDR – Summary
CIDR allows efficient allocation of the IPv4 address
space
o 202.64.28.0/
o 203.72.172.0/
Routing uses the “Longest Prefix Match”
DHCP, ARP, NAT
Internet Addressing – Address
Allocation, Resolution and Translation
