Subnetting Quick Reference guide
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SubnettingSubnetting
Quick Reference guide
SUBNETTING BASICS
• Subnetting is the process of logically dividing a large network address into smaller subnets
• A subnetwork, or subnet, is a distinctly addressed part of a single IP Network.part of a single IP Network.
• Advantages
–Can divide network in smaller parts
–Restrict Broadcast traffic
– Simplified Administration
SUBNETTING BASICS
192.168.2.0/2450 hosts
48 hosts
192.168.1.0/2650 hosts
48 hosts
192.168.1.0/24
Network without subnetting,
too many addresses are wasted
on each subnet
192.168.1.0/26
Network with subnetting,
Addresses are properly
utilized to avoid wastage
SUBNETTING BASICS
• The process of subnetting involves borrowing bits from the network part of a subnet mask and adding it to the host part, thus increasing the network bits and reducing the host bits.
• Subnetting results in prefix lengths greater than the default for any given class, and a subnet mask containing values other than “0” and “255”
192.168.10.0 = 11000000.10101000.00001010.00000001
255.255.255.128 = 11111111.11111111.11111111.10000000
Prefix length = /25
SUBNETTING BASICS• When dividing a network into smaller subnets, we must know
how many parts we are dividing it into.
• Each subnet contains a group of addresses so we’ll need to know which addresses belong to each subnet
• Each network has a network address, a subnet is a small part of a network and should have its own subnet address
• We are able to derive these value using the formulae below• We are able to derive these value using the formulae below– Number of subnets = 2x
Where X = number of bits borrowed from the host part of the subnet mask
– Number of Host addresses in each subnet = 2y-2
Where y = number of 0’s in the subnet mask
– Block Size = 256-Mask
Where Mask is the decimal equivalent of the octet from which bits are borrowed
SUBNETTING CLASS C ADDRESSES• 192 . 168 . 10 . 0 /25
• 11111111 . 11111111 . 11111111 . 10000000
• 255 . 255 . 255 . 128
Network part Host part
• Number of subnets = 2x = 21 = 2 subnets
• Number of hosts per subnet = 2y-2 = 27-2 = 126
• Block Size = 256-Mask = 256-128 = 128
Subnet address 192.168.10.0 192.168.10.128
First valid address 192.168.10.1 192.168.10.129
Last valid address 192.168.10.126 192.168.10.254
Broadcast address 192.168.10.127 192.168.10.255
SUBNETTING CLASS C ADDRESSES• 192 . 168 . 10 . 0 /26
• 11111111 . 11111111 . 11111111 . 11000000
• 255 . 255 . 255 . 192
Network part Host part
• Number of subnets = 2x = 22 = 4 subnets
• Number of hosts per subnet = 2y-2 = 26-2 = 62
• Block Size = 256-Mask = 256-128 = 64
Subnet address 192.168.10.0 192.168.10.64 192.168.10.128
1st valid address 192.168.10.1 192.168.10.65 192.168.10.129
Last valid address 192.168.10.62 192.168.10.126 192.168.10.190
Broadcast address 192.168.10.63 192.168.10.127 192.168.10.191
CIDR MAPPING FOR CLASS C
• 255.255.255.0 : /24 (default)
• 255.255.255.128 : /25
• 255.255.255.192 : /26
• 255.255.255.224 : /27• 255.255.255.224 : /27
• 255.255.255.240 : /28
• 255.255.255.248 : /29
• 255.255.255.252 : /30
CLASS C SUBNETTING SUMMARY
Prefix
notation
Mask Block Size Number of
subnets
Number of valid
Hosts per subnet
/25 128 128 2 126
/26 192 64 4 62
/27 224 32 8 30
/28 240 16 16 14
/29 248 8 32 6
/30 252 4 64 2
Network part Host part
SUBNETTING CLASS B ADDRESSES
• 172 . 12 . 0 . 0 /18
• 11111111 . 11111111 . 11000000 . 00000000
• 255 . 255 . 192 . 0• 255 . 255 . 192 . 0
• Number of subnets = 2x = 22 = 4 subnets
• Number of hosts per subnet = 2y-2 = 214-2 = 16384
• Block Size = 256-Mask = 256-192 = 64
CIDR MAPPING FOR CLASS B
• 255.255.0.0 : /16 (default)
• 255.255.128.0 : /17
• 255.255.192.0 : /18
• 255.255.224.0 : /19• 255.255.224.0 : /19
• 255.255.240.0 : /20
• 255.255.248.0 : /21
• 255.255.252.0 : /22
• 255.255.254.0 : /23
Network part Host part
SUBNETTING CLASS B ADDRESSES WITH CLASS C MASK
• 172 . 12 . 0 . 0 /26
• 11111111 . 11111111 . 11111111 . 11000000
• 255 . 255 . 255 . 192• 255 . 255 . 255 . 192
• Number of subnets = 2x = 210 = 1024 subnets
• Number of hosts per subnet = 2y-2 = 26-2 = 62
• Block Size = 256-Mask = 256-192 = 64
*remember to add the block size to the octet it was derived from*
Network part Host part
SUBNETTING CLASS A ADDRESSES
• 10 . 0 . 0 . 0 /14
• 11111111 . 11111100 . 00000000 . 00000000
• 255 . 252 . 0 . 0• 255 . 252 . 0 . 0
• Number of subnets = 2x = 26 = 64 subnets
• Number of hosts per subnet = 2y-2 = 218-2 = 262142
• Block Size = 256-Mask = 256-252 = 4
*remember to add the block size to the octet it was derived from*
Network part Host part
SUBNETTING CLASS A ADDRESSESWITH CLASS B MASK
• 10 . 0 . 0 . 0 /20
• 11111111 . 11111111 . 11110000 . 00000000
• 255 . 255 . 240 . 0• 255 . 255 . 240 . 0
• Number of subnets = 2x = 212 = 4096 subnets
• Number of hosts per subnet = 2y-2 = 212-2 = 4094
• Block Size = 256-Mask = 256-240 = 16
*remember to add the block size to the octet it was derived from*
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