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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
2 7 26
128 64
1
0
1
1
1
1
1
1
1
1
1
1
1
1
1
1
25
32
0
0
1
1
1
1
1
1
24
16
0
0
0
1
1
1
1
1
23
8
0
0
0
0
1
1
1
1
22
4
0
0
0
0
0
1
1
1
21
2
0
0
0
0
0
0
1
1
20
1
0
0
0
0
0
0
0
1
128
192
224
240
248
252
254
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 255.0.0.0
Class A
• Class B 255.255.0.0
Class B
• Class C 255.255.255.0
Class C
11111111.00000000.00000000.00000000
11111111.11111111.00000000.00000000
11111111.11111111.11111111.00000000
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 32
Bits represents
A network
ID
The remainder is
Used to represent
A 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 Internetconnected 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 A
10? For class B
110 for class C
How Address Classes
Affect a Network
Ranges of 1st octet network ID’s
A
B
C
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
“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
– 255.255.254.0
–
–
–
–
–
–
255.255.255.0
255.255.255.128
255.255.255.192
255.255.255.224
255.255.255.240
255.255.255.248
255.255.255.252
/22 1024 hosts
/23
512 hosts
0
0
128
192
224
240
248
/24
+ 128 /25
+ 64 /26
+ 32 /27
+ 16 /28
+ 8
/29
+ 4
/30
– 255.255.255.254 252 + 2
– 255.255.255.255 254 + 1
– Learn or memorize them.
– Or, use the addition trick in column 2
256 hosts
128 hosts
64 hosts
32 hosts
16 hosts
8 hosts
4 hosts
/31 not usable
/32 single host
Subnetting IP Addresses
• Variable length subnet masks
– Could subnet a class B into several chunks
Network
Network
Host
Subnet
111111111111111111111111
Host
00000000
Subnet
Mask
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 Addressing
Dotted 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?