I am running a home LAN, with ADSL internet access through a D-Link 604+ ADSL wireless router/modem which is connected by Ethernet to a Windows XP computer, and at various times by wireless to another Windows 98 computer and three laptops (running XP and 98). I have allocated fixed IP addresses for each computer with MAC address security implemented on the router. All computers have antivirus software and Zone Alarm firewall. Frequently, one or more of the computers encounters a problem connecting to the internet (using Internet Explorer or Outlook). In each instance the computer transmits packets to the router/modem, but there seems to be few received packets. The problem often arises after being connected for a period of an hour or so. I am able to ping the router, and also have a successful response using ‘net view’. To temporarily fix the problem I must reboot the ADSL router/modem, or restart the affected computer, but this is not successful in all cases. The greatest number of instances of this problem occur when one or more other users in the LAN are running MSN Messenger. I suspect the LAN is somehow being put behind a firewall at the ADSL router/modem, but I have no firewall settings manually selected at the router/modem.

Since you can ping the router, this at least establishes we have connectivity with the router and the device hasn’t crashed. I have a feeling this problem is related to ZoneAlarm being installed on your local machines, which is blocking access to some service (such as DNS) that is required for internet access. Disable ZoneAlarm on the affected local machine and check whether the problem continues. If not, there are a few configuration changes we can make to ZoneAlarm to fix the problem. Try each of these suggestions in order and test to check whether they fix the problem before continuing to the next suggestion. Firstly, make sure you are running the latest version of ZoneAlarm. Secondly, open ZoneAlarm and go to the ‘Firewall’ section. On the ‘Main’ tab, click the ‘Advanced’ button. Tick ‘Allow Outgoing DNS/DHCP’ in Internet Zone on High setting’ (you will notice the setting for Trusted Zone is also automatically ticked - this is normal). Click OK. Thirdly, click the ‘Firewall’ tab and click the ‘Zones’ tab. Click the ‘Add >>’ button > ‘IP Address’. In ‘Zone’, select ‘Trusted’ and for IP Address type ‘127.0.0.1′. For the description, write ‘Localhost’. Click OK. Change to the ‘Overview’ section and click ‘Yes’ when prompted to save changes. Fourthly, repeat the previous procedure but enter the private fixed IP address of your ADSL router and name the description ‘Router’. I don’t think that MSN Messenger would be directly causing the problem, but it’s possible that the network traffic being generated could be triggering the primary cause of the issue. Please let me know if these suggestions fix your problem, as it would be useful to know for future reference.

As a side note, some people believe that being behind a router provides enough protection against attack, so there is no need to run a firewall on the local machines. Most routers do provide a basic level of protection through methods such as NAT (Network Address Translation) and SPI (Stateful Packet Inspection). However, for more advanced protection (including features such as outbound program control) having a correctly configured firewall on your local machine will provide a greater degree of protection.