Why create a subnet mask?

• Tells device which part of an address is the network # including the subnet and which part is the host

• Filters the node IP address to determine which subnet it belongs to

• NETWORK/SUBNET/HOST

How to Create Subnets 27 26 25 24 23 22 21 20

128 64 32 16 8 4 2 1

1 0 0 0 0 0 0 0 128

1 1 0 0 0 0 0 0 192

1 1 1 0 0 0 0 0 224

1 1 1 1 0 0 0 0 240

1 1 1 1 1 0 0 0 248

1 1 1 1 1 1 0 0 252

1 1 1 1 1 1 1 0 254

1 1 1 1 1 1 1 1 255

The class determines which part of the address belongs to the

network and which part belongs to the host

Class A

NNNNNNNN.hhhhhhhh.hhhhhhhh.hhhhhhhh

Class B

NNNNNNNN.NNNNNNNN.hhhhhhhh.hhhhhhhh

Class C

NNNNNNNN.NNNNNNNN.NNNNNNNN.hhhhhhhh

IP Subnet addressing default subnet masks

In Binary Form

Class A

11111111.00000000.00000000.00000000

Class B

11111111.11111111.00000000.00000000

Class C

11111111.11111111.11111111.00000000

• Class A -

255.0.0.0

• Class B - 255.255.0.0

• Class C - 255.255.255.0

Restrictions on subnets

• Network addresses of all 0’s in the host portion are reserved for specifying the network

• Network addresses of all 1’s in the host portions are reserved for the broadcast address

How an IP address Is Composed

Part of the 32Bits representsA networkID

The remainder isUsed to

representA host with the

network

Network & Host IDs• Network ID – Each network has a unique

network number– Each Network connected to the Internet has to

have a globally unique ID – no other Internet-connected network in the world can have the same Network ID

• Host ID – – Within a given network – Host IDs are used to identify

hosts • Hosts – any device that needs to be addressed by an IP address

- computers, printers, routers, etc.

– Host IDs must be unique within a given network.

How Bits Are Set Up for Each IP Address Class

Note – This shows the binary values in the first 3 bits of the 3 classes:0?? For class A10? For class B110 for class C

How Address Classes Affect a Network

0 0 0 0 0 0 0 1

1 0 0 0 0 0 0 0

1 1 0 0 0 0 0 0

0 1 1 1 1 1 1 0

1 0 1 1 1 1 1 1

1 1 0 1 1 1 1 1

Ranges of 1st octet network ID’s

A

BC

“private” address – cannot be routed on the internet

IP Address Guidelines• First Octet Network ID Rules

– Network ID cannot be 127.– ID bits cannot be all “1”s.– ID bits cannot be all “0”s.

• For class B or C Network IDs– Second octet (and the third octet for class C

networks) can be any number from 1 -255, or 00000000 to 11111111

• Class B: 131.0.x.x or 131.255.x.x are OK• Class C: 200.0.0.x or 200.255.255.x or

200.255.0 are all OK

IP Address Guidelines• First Octet Network ID Rules

– Network ID cannot be 127.– ID bits cannot be all “1”s.– ID bits cannot be all “0”s.

• For class B or C Network IDs– Second octet (and the third octet for class C

networks) can be any number from 1 -255, or 00000000 to 11111111

• Class B: 131.0.x.x or 131.255.x.x are OK• Class C: 200.0.0.x or 200.255.255.x or

200.255.0 are all OK

Problems with default masks

• They only provide for a single network segment– Class C – 255.255.255.0 allows for a maximum of 254 hosts on the

segment– Class B – 255.255.0.0 allows for a maximum of 65,534 hosts on the

segment– Class C – 255.0.0.0 allows for a maximum of 16,777,214 hosts on

the segment– Beyond class C networks, current LAN technologies will not support

anywhere near the maximum number of hosts/segment

• Since there is only 1 network segment:– Difficult to use different topologies in the LAN (Ethernet, FDDI,

Token Ring)– Difficult to have a geographically dispersed LAN connected using a

WAN technology.

Common masks• Masks

– 255.255.252.0              /22 1024 hosts– 255.255.254.0             /23  512 hosts

– 255.255.255.0   0         /24  256 hosts– 255.255.255.128 0   + 128 /25  128 hosts– 255.255.255.192 128 + 64  /26   64 hosts– 255.255.255.224 192 + 32  /27   32 hosts– 255.255.255.240 224 + 16  /28   16 hosts– 255.255.255.248 240 + 8   /29    8 hosts255.255.255.252 248 + 4   /30    4 hosts

– 255.255.255.254 252 + 2   /31 not usable– 255.255.255.255 254 + 1   /32 single host– Learn or memorize them.– Or, use the addition trick in column 2

Subnetting IP Addresses

• Variable length subnet masks– Could subnet a class B into several chunks

Network Host

Network HostSubnet

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0SubnetMask

Problem #1: Lifetime of Address Space

• Example: an organization needs 500 addresses. A single class C address not enough (256 hosts). Instead a class B address is allocated. (~64K hosts) That’s overkill -a huge waste.

IPv4 AddressingDotted Decimal Notation

• Dotted Decimal Notation– Four bytes (8 bits = 1 byte) per address– Each byte separated by a dot– Each byte expressed in decimal notation

• Example: – Dotted Decimal Notation: 192.16.224.254– Binary Notation?:

– What is the minimum decimal value any byte can be assigned?

– What is the maximum decimal value any byte can be assigned?