The GUI may limit you to setting only five IP addresses, but according to Microsoft "Windows NT 4.0 has no limitation on the number of IP addresses that can be bound to a NIC. The limitation is on system resources."
The Microsoft Knowledge Base Article given below is entitled "Adding More than Five IP Addresses to NIC in Windows NT."
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I have seen posts by admins on a BIND NT mailing list claiming to have hundreds of IPs bound to single NICs.
The most I have had bound to a single NIC is about a dozen - I didn't have a reason to try more.
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