Data Networking Year 2

IP Addressing Revisited

Colm Bennett

IP Addressing

32 Bit number, 4 decimal parts, e.g.192.168.1.0

Unique Host Address

Subnet Mask

Identifies which part of the IP address is the Network Address and which is the Host Address

Default Gateway

IP Address of the router on the same physical segment

An IP address is a 32 bit binary number usually represented as 4 decimal values, each representing 8 bits, in the range 0 to 255 (known as octets) separated by decimal points. This is known as "dotted decimal" notation.

What is a host?

Network/Host

Network ID

Same for all hosts on the one physical segment

Unique within the overall network

Host ID

Identifies a particular device within this segment

Unique within the segment

Physical Segment

Broadcast domain

i.e. The part of the network that will respond to a broadcast packet

So repeaters, bridges, switches etc all included as they forward on broadcasts

But routers don't so a segment is...

All devices out of one port of a router

All devices between router

All devices have same network ID and a unique Host ID

Classes

Class A

First Octet 0-127

Network.Host.Host.Host

Class B

First Octet 128-191

Network.Network.Host.Host

Class C

First Octet 192-223

Network.Network.Network.Host

D and E are for special uses

This was arbitary no particular reason it was done like this.Most companies deal with Class C This was how subnetting was done originally purely by looking at the first octet very wasteful!

Classes

Easy way to remember

Each class is half the size of the previous

256 possible values for the first Octet

Half these are given to class A

1-127

Half the remaining 128 (64) are given to B

128-191

Half the remaining 64 (32) are given to C

192-223

And again for D and E

Private Address Spaces

Class A

10.0.0.0

Class B

16 Class B's starting from 172.16.0.0

Class C

256 Class C's starting from 192.168.0.0

Subnet Mask

Subnet Mask is 32 bit number that indicates the network and host parts of an IP address

1's indicate the network part, 0's the host part

Doesn't have any real meaning in isolation

Only makes sense when used with an IP address

What are the subnet masks implied in the standard Class A, B & C networks?

This was introduced cos the normal classes were so wasteful

Going from A,B,C to always specifiying a subnet

Allows us to move to actual subnetting

Alternative Notation

Slash notation puts a number after the IP Address indicating how many bits will be used for the network address

Sometimes called the Prefix as it indicates the IP address is prefixed with a certain number of network bits

So 192.168.5.0 /24 means the first 24 bits are the network part

Same as subnet mask of 255.255.255.0

How Many IDs for each class?

Network Ids

Simply take the total maximum number of the network part for that Class

Exception are two class A's not available 0 and 127

127 is reserved for Loopback and diagnostics

e.g. Class C 192.0.0.0 - 223.255.255.255

So max number of network Ids is 32*256*256

2,097,152

How Many Ids for each class?

Host Ids

Don't use all 0's or all 1's host Ids

All 0's identifies the network

All 1's is the network broadcast address

So always take 2 off the possible hosts

Class C (255.255.255.0) has the last octet for hosts

So 256 2 = 254

Other Classes?