I’ve recently dealt with 2 customers who were wanting to have 3 or 4 computers share a single internet connection via an ethernet switch.

In most cases, I believe that this cannot be done easily. there are 4 ways I can think of:

1. Get the ISP to allocate an IP address to each PC (this can start getting expensive… as most ISP‘s will charge a monthly fee for each extra IP address. But at least each PC will have its own connection to the internet (and each PC should have its own firewall and anti-malware systems).
2. Connect the modem usb cable (if there is one) to a PC (which needs to always remain running). Connect this PC (lets call it PC1) to the switch via ethernet (and setup the ethernet to have a fixed IP address). Bridge the usb and ethernet connections together. Then give all the other PCs an IP address in the same ballpark as the ethernet on PC1. Cons: If PC1 shuts down, everyone else loses internet connectivity.
3. Use a PC with 2 ethernet ports. This is similar to point 2., except you substitute the usb connection with a second ethernet connection.
4. Setup a dedicated “router” PC. A router PC is not as reliable as a pure hardware router, and is more expensive/time consuming to setup and maintain.

In both cases, I suggested that the customers get a router. Its more secure, each user accesses the internet independently, no problems with fixed IP addresses, its cheap, and is generally a more flexible solution.