Setting up a secondary IP address range


We are running out of IP's in our DHCP range and do not have 'room' in our IPv4 to enlarge the current scope. Can you setup a secondary IP range to use DHCP?

Like so:

interface vlan.0.100
ip address 10.10.1.0 255.255.255.0 primary
ip address 10.10.7.0 255.255.255.0 secondary

8 replies

Userlevel 4
Hi Jeremy,
working with secondary IP addresses on a router interface is possible but alway a nasty workaround.

Regards
André
André Herkenrath wrote:

Hi Jeremy,
working with secondary IP addresses on a router interface is possible but alway a nasty workaround.

Regards
André

Why 'nasty'?
Userlevel 4
André Herkenrath wrote:

Hi Jeremy,
working with secondary IP addresses on a router interface is possible but alway a nasty workaround.

Regards
André

Both subnets are broadcastdomains - usually broadcastdomains are separated by routers - you plan to connect them with a bridge.

You better create a second vlan with its own IP range and use tagging on the uplinks.

/André
Userlevel 4
André Herkenrath wrote:

Hi Jeremy,
working with secondary IP addresses on a router interface is possible but alway a nasty workaround.

Regards
André

I am not sure I would use the adjective "nasty". Adding the secondary address with another class c address is not much different than backing off the mask by 1 bit. That said I understand and agree with the sentiment of keeping broadcast domains as small a possible.
Userlevel 4
André Herkenrath wrote:

Hi Jeremy,
working with secondary IP addresses on a router interface is possible but alway a nasty workaround.

Regards
André

When it stops with the first secondary IP - I agree. But you are opening the door to a bad style of networking (I have customers with 12 secondary IPs in a subnet). Creating a second vlan shouldn't be a problem.
Userlevel 7
André Herkenrath wrote:

Hi Jeremy,
working with secondary IP addresses on a router interface is possible but alway a nasty workaround.

Regards
André

Another thing is the DHCP request - I assume DHCP relay will be used,
In that case the router will send out 2 requests for the different scopes and the first reply will decide which IP (subnet) is used by the client.

In my opinion it's better to configure a new VLAN instead of using a secondary IP.
Userlevel 4
André Herkenrath wrote:

Hi Jeremy,
working with secondary IP addresses on a router interface is possible but alway a nasty workaround.

Regards
André

Your DHCP server will need to be configured to show the different subnets are actually a part of the same network. Our BlueCat DHCP servers are able to do that.
Userlevel 7
Hi Jeremy,

you can add a secondary IP address to a VLAN:
configure vlan <NAME> add secondary-ipaddress <ADDRESS> [/code]You can configure the BOOTP Relay service to include the secondary address(es):
configure bootprelay ipv4 include-secondary [/code]Br,
Erik

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