1.8 IPv4 Subnetting

Determine requirements

1. Determine the number of required network ID's
2. Determine the number of host ID's per subnet
3. Create the Subnets

Subnet mask- 32 bit value that allows the receiving host to determine the Host ID from the network ID portion of the IP address.

Class A subnet - Network.node.node.node - 255.0.0.0

Class B subnet- Network.Network.Node. Node- 255.255.0.0

Class C subnet- Network.Network.Network.Node- 255.255.255.0

CIDR- Classless Inter-Domain Routing- Used by ISP's to apply a block of IP's. Signified by IP address/X. The X signifies the number of 1's in the subnet mask. The largest subnet mask availible is a /30. This is due to needing to keep 2 bits open for the host bits.

IP subnet-zero- this command allows you to use the first and last subnet in your network design. Without it you can not use these.

Powers of 2- Need to memorize

2^1=2

2^2=4

2^3=8

2^4=16

2^5=32

2^6=64

2^7=128

2^8=256

2^9=512

2^10=1024

2^11=2048

2^12=4096

2^13=8192

2^14=16384

CIDR Values

255.0.0.0 = /8

255.128.0.0 = /9

255.192.0.0 = /10

255.224.0.0 = /11

255.240.0.0 = /12

255.248.0.0 = /13

255.252.0.0 = /14

255.254.0.0 = /15

255.255.0.0 = /16

255.255.128.0 = /17

255.255.192.0 = /18

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

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

255.255.255.254 = /31

255.255.255.255 = /32

Subnetting the easy way

• How many subnets- 2^x= number of subnets- x= the number of 1's in the host bits.
• How many hosts per subnet- 2^y-2 = Number of hosts per subnet- Y= the number of 0's in the host bits. -2 to accommodate for the host and broadcast bits.
• What are the valid subnet masks- 256- subnet mask = Block size- Start at 0, add the block size up till you reach the subnet mask value, each is a block.
• Whats the broadcast address for each subnet- This is always the last IP in the block.
• What are the Valid Hosts- The valid hosts address are between the first address and the broadcast address not including those.

Variable Length Subnet Masks(VLSM)

• Used with RIPv2, EIGRP, OSPF
• Classless routing-
• Classful- All subnets are the same and each VLan has the same possible number of hosts
• Means you can use subnet masks of different lengths for different router interfaces

Summarization

• Allows routers to advertise multiple routes as a single route.
• Allows for smaller routing tables.
• Limits overhead on a network