When I installed a new firewall at a site, the domain controller could not ping the firewall nor access the Internet through the firewall, yet it could ping and access all other systems on the LAN. The problem was due to the domain controller using a SiS 900-based PCI Fast Ethernet network adapter, which had a physical, aka MAC, address of all zeroes. When I assigned a non-zero address to the adapter, the problem disappeared.
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