Hello. I'm having a little problem with configuring the firewall on my server. I only want a specific range of IPs to be able to access specific ports. For example, I'm having alot of password attempts to some of my servers, so I want to make it more safe by only allowing incoming connections from a specific range of domain.

Example:

My IP is usually adsl-324-4.somecompany.com so I want to allow *.somecpompany.com to connect, as my IP is dynamic. That would get rid of alot of attempts to hack into my servers. But I have no idea how to mask a domain like that for the firewall. How could I for example allow all incoming connections from *.is?

Thanks.