If you have compiled your kernel with the IPFIREWALL option, you need to be aware that the default policy as of 2.1.7R (this actually changed during 2.1-STABLE development) is to deny all packets that are not explicitly allowed.
If you had unintentionally misconfigured your system for firewalling, you can restore network operability by typing the following while logged in as root:
# ipfw add 65534 allow all from any to any
You can also set firewall_type="open" in /etc/rc.conf.
For further information on configuring a FreeBSD firewall, see the Handbook section.