Offtopic: I read on DSLreports/Broadbandreports that if you have Comcast, you're forced to use their modems because the static IP addresse is configured into them and they don't share the password with you. How did you get to use your own modem?
I think I was able to use my own modem because a) neither of the modems brought by the service tech would work b) I had a modem that was known to work on their network. I was lucky. The service tech called their office to configured their system to work with my modem. It seemed all they really had to do was know the MAC address (or something similar) printed on the bottom of the modem.
I have the lowest level of business service. It does not include a fixed IP. I don't know exactly how the modem works on their network, but on my small office LAN, fixed IP is set by the router using the MAC address of the each device. The router running DHCP always gives the same IP to each device I have specified to have a fixed IP. Everything else has a dynamic IP.
I use Dyndns.com with my router, a Linksys model with Tomato firmware installed. The firmware notices when the IP changes and sends a message to dyn.com where my domain IP address is updated. For about $2.50/month, I effectively have a fixed IP, if my home-office network is contacted via the domain name I have chosen. My home-office LAN is described at http://www.bobino.me/greenside-network.
The modem they tried to install was also a router/NAT firewall with DHCP/DNS service. It is doubtful I could have configured it as I do my own router/firewall to allow passage of selected ports and to use an alternative DNS. I was pretty unhappy the day as they tried to install their modem until they couldn't get either of them to work.
The lowest cost Comcast business service costs me $15 a month more than than the residential service I had previously. The biggest difference is that I am not bandwidth limited using the business service. BTW, you don't want to go over their bandwidth caps for residential service. The consequences are very punitive: no internet for 6 months! They treat their customers like children.