# Lecture 17: Network Layer: Logical addressing

Δίκτυα και Επικοινωνίες

24 Οκτ 2013 (πριν από 4 χρόνια και 8 μήνες)

77 εμφανίσεις

NETWORK LAYER

Ch

For world
-
wide communications

Uniquely identify a device in the network

32 bits long

2
32

Globally unique

Notation (binary or dotted
-
decimal)

Classful

Address space is divided into five classes: A, B, C, D, and E

netid

hostid

Classful

Mask is used to find the
netid

and
hostid

Subnetting

divide a large block of addresses into
several groups (
increases the number of 1s in the mask
)

Supernetting

combine small blocks of addresses due
to depletion of class A and B addresses.

A large part of addresses were wasted

used.

Allow a different size of address blocks with restrictions
of

Addresses in a block must be
contiguous

The number of addresses in a block is
a power of 2

divisible

by the number of

A block of addresses can be defined as

x.y.z.t /n

where x.y.z.t is

/
n defines

First n
bits are the
netid

The block can have up to 2
(32

n)

hosts

Practice with example 19.9

Exp
: 205.16.37.39/28 will be represented in binary as

11001101 00010000 00100101 0010
0111

(205.16.37.32 is the first address in the block)

The first address is often assigned to a special device, e.g., a gateway

Comparison

Telephone number

Subnetting

(Multi
-
level Hierarchy)

“Internally” divide a block of addresses into
clusters of networks (called subnets)

An outside device still sees them as a single block

Example: Organization is given

17.12.14.0/26

00
0
00000

00
0
11111

00
10
0000

00
10
1111

00
11
0000

00
11
1111

17.12.14.

17.12.14.

17.12.14.

Needs to split in 3 groups.

Example 19.10

A block of addresses 190.100.0.0/16 needs to be distributed to three
groups as follows:

64 customers; each needs 256 addresses

128 customers; each needs 128 addresses

128 customers; each needs 64 addresses

Home users have a small network, where each device

Some addresses are reserved for “internal network”

Anyone can use these addresses without permission

NAT

The private network should has one single
connection to the Internet

Translation Table

Translation Table

Entry of a table may …

use a pool of (global) IP addresses

often used by
ISP (see below)

use both IP address and port number

often
known as IP sharing

NAT and ISP

IPv6

128
bits long

2
128

Globally
unique

Abbreviated representation

Unicast

Multicast

Homework

Exercise

18

19

22

26