Other hubs remember Ethernet address seen at certain ports and can be configured to stop access for new Ethernet addresses. Can be useful. Bridges Useful for breaking up subnets into small segments, making it easier to localise errors. Restricts traffic local to machines to that segment, by sensing what ethernet addresses are where. Where NIS is used, allowing IP broadcast across subnets also decreases security.
Routers have a configuration port, often accessible via telnet.
Analyse this logs regularly. This can then be checked against a list of registered modems. Modems which are only needed for outgoing calls should be configured to ignore incoming calls. The user should prove that such an external access is absolutely necessary. Dialup access for company partners.