Transcript IP Addressing

IP Addressing
IP Addressing
• A 32-bit logical naming convention
• A dotted-decimal notation is used:
– 170.56.70.1
– Each number represents 8 bits. Number is 0-255
• Part denotes the network and part denotes the
client on the network
• IP address must be unique and clients on the same
network must have the same network ID
• IP Classes
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Class A- Starts with 0, range of 1-127
Class B – Starts with 10, range of 128 - 191
Class C – Starts with 110, range of 192 - 223
Class D – Starts with 1110, range of 224 – 239
Class E – Starts with 1111, range 240 – 255
IP Addressing Contiuned
• IP address come pared with a subnet mask
• There is a default subnet mask for each IP class
– Class A 255.0.0.0
– Class B 255.255.0.0
– Class C 255.255.255.0
• The subnet mask tells what part of the IP address
denotes the network and which part denotes the
client
Division of Address Space
• The IP class scheme does not divide the 32-bit
address space into equal size classes
Authority for Addresses
• An organization obtains its IP network numbers
from an Internet Service Provider (ISP).
• ISPs coordinate with a central organization, the
Internet Assigned Number Authority, to ensure
that each network prefix is unique throughout the
entire Internet.
A Classful Example
• When assigning a network prefix, a number must
be chosen from class A, B, or C
• Usually networks are assigned a class C address
• For networks connected to the global Internet, a
service provider makes the choice.
• For a private network, the local network
administrator selects the class
A Classful Example
Classless Addressing
• The original classful scheme was limiting
• IP addresses were being exhausted
• Many assigned network numbers caused many
addresses to be unused
• New mechanisms were developed:
– Subnet addressing
– Classless addressing
• Idea: Instead of having three distinct address
classes, allow the division between prefix and
suffix to occur at any bit
Why Classless Addressing Helps
• Consider a network that contains 9 hosts
• Only 4 bits of host suffix are needed to represent
all possible host values.
• However, a class C address has names for 256
hosts.
• Classless addressing solves this problem by
allowing an ISP to assign a prefix that is 28 bits
long and the suffix then is 4 bits long.
• With 4 bits for the suffix, there are 16-2 = 14 host
numbers
Address Masks
• Classless addressing requires an additional
piece of information with each address
• This additional information specifies the
exact boundary between the network prefix
and the host suffix.
• Thus with classless addressing, tables inside
hosts and routers must contain two pieces of
information: the 32-bit address and another
32 bit value that specifies the boundary
between the prefix and the suffix (subnet
Address Masks Continued
• The subnet mask is stored as a 32-bit binary
number
• This allows for efficiency in extracting the
network part of the IP address by the host or
router
• The prefix portion of the IP address needs
to be compared to values in the routing
table.
• This can be done in two machine
How Routing Table Comparison
is Done
• Suppose D is a packet final destination
• Suppose (A, M) represents a 32-bit network
Address and a 32-bit Mask entry in the
routing table
• The router tests the condition:
Is A = (D &M) ?
• M = 255.255.0.0
• A = 128.10.0.0
• D = 128.10.2.3
CIDR Notation
• 128.10.0.0/16
• This notation specifies that the first 16 bits
of the address make up the network prefix
CIDR Address Block Example
• If an ISP has two computers each with only
12 computers each, the ISP can use CIDR to
partition a class B IP address into three
parts: two for each of the customers and the
remainder available for future use.
• One customer can be assigned:
128.211.0.16/28
• The other customer can be assigned:
128.211.0.32/28
CIDR Host Addresses
Special IP Addresses
Routers and IP Addressing
Principle
• Routers are assigned IP addresses as well as
host machines
• A router may be assigned two or more IP
addresses