Two ADSL modems on one LAN

At the moment we have two buildings each with their own LAN's and each with their own internet connection through an ADSL modem, using their own seperate telephone line. Next week we are going to connect up ther buildings using a laser link. The whole company will then be on one LAN.

The only problem is that both buildings have at least 18 month left on the internet contracts. Can I still use both modems on 1 LAN? If so will this give me more bandwidth?

You can leave things exactly as they are if you want, there's no harm - but no easy way of using both links though. The other thing to remember is that those laser links, though great, are not 100% reliable, so leaving the existing setup in place means people would have the same connectivity they have today if it got foggy/snowy.

You best bet may be to keep both links, and (either with DHCP or static configuration) give machines in each building their "local" router as a default gateway. This avoids Chopper3's highlighted issue whereby people could lose Internet access if the laser links fail, and gives you very low-tech load balancing to utilise both connections.

Alternatively, if you have a service that tends to saturate your ADSL uplink (say), or a service that requires low latency (VOIP?), you might choose to just route specific services over one link -- for instance, at one of our sites I have VOIP handsets talking to a controller at our "main" site, these are on the same subnet as all our PCs, servers etc I have 2 ADSL routers, as 192.168.0.253 and 192.168.0.254, VOIP goes through the former and all other Internet traffic through the latter.

A final option would be to use one link as the "primary" and fail people over to the other if it goes down, by switching its IP to their configured default gateway. This would give you again a fairly low-tech approach to increasing the availability of ADSL, which ... well your mileage may vary, but I'm always disappointed :-)

Really it depends what your needs are -- do you need to increase bandwidth/availability for your users in general, do you have specific services that need their own private lane for traffic (either to avoid impacting others, or to avoid being impacted), or are you really just OK as you are.