Can ASA5505's DHCP support 22 bits subnet mask?

[tinhnho123]

Hello,

I have an ASA5505 which provides internet (just internet) for about more than 600 pc/laptops. Can 5505's DHCP support this number?

Thanks.

[Jouni Forss]

Hi,

Sadly the ASA only supports /24 size DHCP Pool

We have faced this same problem in some cases. (Although very rarely need to use ASA as a DHCP server)

Quote from ASA Command reference

The size of the address pool is limited to 256 addresses per pool on the ASA. If the address pool range
is larger than 253 addresses, the netmask of the ASA interface cannot be a Class C address (for example,
255.255.255.0) and needs to be something larger, for example, 255.255.254.0.

- Jouni

[Jouni Forss]

If you have a possibility to divide the users accross multiple interfaces on the ASA then naturally you could configure DHCP pools for each interface and this way be able to use the ASA alone for DHCP services.

If you are limited by the amount of Physical interfaces you could consider configuring a Trunk between ASA and Switches to only use one physical interface while still enabling multiple subinterfaces on the ASA where you can use a /24 DHCP pool in each.

- Jouni

[tinhnho123]

I guess I'll need an external DHCP server for 22 bits subnet mask then?

[Jouni Forss]

If you want to configure a single DHCP Pool larger than /24 addresses then yes you need something else than an ASA to handle that. (If configuring multiple interfaces on the ASA isnt an option and I guess it is far from ideal)

Either a server or perhaps even Cisco router

- Jouni