4. Easy Subnetting Flashcards
Convert binary value
10000000
128
Convert binary value
11000000
192
Convert binary value
11100000
224
Convert binary value
11110000
240
Convert binary value
11111100
252
Convert binary value
11111110
254
Convert binary value
11111111
255
Memorise
10000000
11000000
11100000
11110000
11111000
11111100
11111110
11111111
10000000 128
11000000 192
11100000 224
11110000 240
11111000 248
11111100 252
11111110 254
11111111 255
Convert binary value
11111000
248
What is the purpose of a subnet mask?
A subnet mask allows the host bits of an IP address to be divided into logical groups that are identified by the subnet mask
What are the different address classes How do you identify them (and their ranges)?
What are the default subnet masks?
IP address classes are A, B and C
They are identified by the first digit in the IP address, i.e.
Class A = 0xxx xxxx (0 to 127)
Class B = 10xx xxxx (128 to 191)
Class C = 110x xxxx (192 to 223)
Class A network.node.node.node
Class B network.network.node.node
Class C network.network.network.node
Default subnet masks:
Class A 255.0.0.0
Class B 255.255.0.0
Class C 255.255.255.0
Powers of 2 are useful
Memorise
2 power 8 onwards:
256 512 1024 2048 4096 8192 16384
What is CIDR?
Classless Inter-Domain Routing
i.e
255.0.0.0 /8
255.255.0.0 /16
255.255.254.0 /23
255.255.255.0 /24
255.255.255.128 /25
255.255.255.192 /26
255.255.255.224 /27
255.255.255.240 /28
255.255.255.248 /29
255.255.255.252 /30
Convert binary value
11111000
248
How do you calculate a Class C subnet for 192.168.10.21 /28?
Look at 1st digit of address (192)
This is a C class address so host bits are contained in the 4th byte and these are the ones we will be dividing up into subnets.
The other 3 digits (192.168.10) will remain unchanged
Calculate subnet bits i.e. 8+8+8+?(4)=28
Bit mask for 4th byte is 1111 0000
128+64+32+16=240
Subnet mask is 255.255.255.240
Block size is lowest (1) bit in the mask 1111 0000 i.e. 16
Subnets will count up 0, 16, 32, 64, 80, 96, 112, 128… 224, 240
0000 (4) host bits would give 16 addresses
First address is used for subnet
Last address is used for broadcast
Addresses - 2 = 14 hosts
Subnets are:
192.168.10.0
192.168.10.16
192.168.10.32
Our address is in the second block so
Subnet address is 192.168.10.16
First valid address 192.168.10.17
Last valid address 192.168.10.30
Broadcast address 192.168.10.31
How do you calculate a Class B subnet for 172.16.10.21 /17?
Look at 1st digit of address (172)
This is a B class address so host bits are contained in the 3rd & 4th bytes and these are the ones we will be dividing up into subnets.
The other 2 digits (172.16) will remain unchanged
Calculate subnet bits i.e. 8+8+?(1)=17
Bit mask for 3rd byte is 1000 0000
128+0=128
Block size is lowest (1) bit in the mask 1000 0000 i.e. 128
Subnets will be 0, 128
Subnet mask is 255.255.128.0
15 host bits (7 in 3rd, 8 in 4th) = 32,768 addresses (2**15)
First address is used for subnet
Last address is used for broadcast
Addresses - 2 = 32,766 hosts
Subnets are:
172.16.0.0
172.16.128.0
Our address is in the first block so
Subnet address is 172.16.0.0
First valid address 172.16.0.1
Last valid address 172.16.127.254
Broadcast address 172.16.127.255
What does the setting IP Subnet-Zero do?
This allows the use of the first and last subnet.
For example with IP Subnet-Zero enabled a Class C mask of 255.255.255.192 provides subnets 0, 64, 128, 192
With it disabled the subnets are 64 and 128
