Why can't a domain controller be found in other subnets?

Hi,

I am not sure how to ask this question so I will give an example.

Here we have many subnets 10.9.0.0 / 16   all broken down to /24 networks.

Domain Controllers live on 10.9.9.0/24

When a host is on the same subnet and I go to join the domain I can type the short name of the domain and it will then ask for credentials to join the domain, "mydomain"

Cool.

When a host is on another subnet, I must type the fully qualified domain name...   mydomain.company.local

I've put up with it for years, now I'd like to know why it happens and what to do so I can type the short name from now on.

Thanks!

Dane!