| | Subnetting
Important: CCNA now uses the ip subnet-zero command, so with a subnet
mask of 192 the subnets 0 and 192 are useable in addition of the subnets 64 and
128.
Subnetting without VLSM is used for classful routing (all nodes use same net
mask), with VLSM, classless routing is necessary.
Procedure for subnet planning:
- Number of subnets
One per subnet
One per WAN-link
- Number of host IDs per subnet
One per TCP/IP host
One per router interface
- when these figures are known, calculate the following numbers:
a single subnet mask for the whole network
a subnet ID for every subnet (physical segment)
Host ID range for each subnet
|
Subnet mask |
CIDR Value |
|
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 |
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