Lan connection to separate building

You say the distance is "about 100 meters" you to know the exact distance as TP links are limited to 100 meters. If you inside the limit you can use a switch at both ends of the link and go another 100 meters from it. If the distance is over 100 butunder 300 meters you could use a 10base2 (coax) link and be limited to 10 mbps. The alternative is fiber, pricey but fast and long. Another option is wireless, slow and subject to interferance.

You need to know the existing "circuit length" and the proposed circuit lenght. A switch will reset your wiring budget for the next hop. Assume that the circuit length from the building perimiter to the server or last switch is 30 meters (arbitrary)and the circuit length to the next switch in the other building is 100 meters, if you could place a switch midway (65 meters) it would work but would only allow an additional 35 meters to the next switch or NIC. If the midpoint is in a field or parking lot this is not practical. You need a device to reset the wiring budget every 100 meters. So if you place a switch at the exit point from the first building (30 meters) and another switch at the entry point of the second building (100 meters) youthen have a "new budget" of 100 meters to connect to the other LAN. DO NOT confuse geographical distance with circuit length, it can take 100' of wire to connect to a device 50' away before reaching that device. You must consider the physical limits, existing circuit length and the circuit requirements at the destination, before you choose a LAN circuit architecture. Another factor is bandwidth requirements for the combined LANs. SMC has an excellent disertation on network architecture on theirweb site with all the limitations of the various methods and suggestions to make it work.