For an Internet site, decide how many peak concurrent users you need the server to handle, and multiply that number of users by the average request size on your site. Your average request can include multiple documents. If you are not sure, try using your home page and all of its associated subframes and graphics.
Next decide how long the average user will be willing to wait for a document, at peak utilization. Divide by that number of seconds. The result is the WAN bandwidth your server needs.
For example, to support a peak of 50 users with an average document size of 24 KB, and to transfer each document in an average of 5 seconds, 240 KBs (1920 Kbit/s) are needed. Therefore, this site needs two T1 lines (each 1544 Kbit/s). This amount of bandwidth also allows some overhead for growth.
Your server’s network interface card is intended to support more than the WAN to which it is connected. For example, if you have up to three T1 lines, one 10BaseT interface will be adequate. If you have up to a T3 line (45 Mbit/s), you can use 100BaseT. But if you have more than 50 Mbit/s of WAN bandwidth, consider configuring multiple 100BaseT interfaces, or look at Gigabit Ethernet technology.
For an intranet site, your network is unlikely to be a bottleneck. However, you can use the same calculations above to verify this.