Secondary IP subnet allocation for better IP management.

Posted by: shahnawazkhot on: February 7, 2012

• In: Switching
• Leave a Comment

In any local area or campus area network, it is always a need to manage a VLAN subnet block with sets of static and dynamic IP address allocations.

Every user VLAN has users having dynamically assigned IP addresses whereas printers, plotters, network scanners are the devices normally mapped into the same user VLAN subnet but having static IP addresses.

The general practice seen in networking world is to exclude few IP addresses from defined DHCP scope for users. These excluded IP addresses are actually reserved for and are assigned statically to printers, plotters and network scanners.

The definition and management of static and dynamic IP address blocks in a VLAN subnet does require an extra administrative work. Sometime, it also leads to IP address conflicts in big local area or campus area networks where the communication is not properly done between network administrators and printer support team who has given the responsibility to allocate static IP addresses to printers, plotters etc.

I personally think of having allocation of two IP subnets to every VLAN. These subnets should be defined with a systematic approach to avoid any wastage in IP addresses. The primary subnet should be allocated to user VLAN whereas the secondary subnet can be used for static IP assignment to printers, plotters and network scanners.

The primary subnet should be given to DHCP administration team to define the DHCP pool whereas the secondary subnet is managed for static IP address allocations.

We always have dynamic user IP block bigger than the static IP address requirement for printers, plotters etc.

Let’s consider that /24 subnet mask is for user vlan subnet and /28 subnet mask is for printers, plotters and network scanners etc.

The configuration of primary and secondary IP subnets to physical or vlan interface is a very simple task in Cisco IOS. The example is as mentioned below.

Switch(config)#int fa0/0   –>  (vlan interface can also be configured for secondary IP address)

Switch(config-if)#ip address 10.10.10.1 255.255.255.0       –>    Primary subnet configuration.

Switch(config-if)#ip address 10.10.11.1 255.255.255.240 secondary  –> Secondary subnet configuration.

We can test the reachability of the configured primary and secondary IP subnets in the network as following.

Switch#ping 10.10.10.1

Sending 5, 100-byte ICMP Echos to 10.10.10.1, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/4 ms

 

switch#ping 10.10.11.1

Sending 5, 100-byte ICMP Echos to 10.10.11.1, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/4 ms