This 8-byte bitmask specifies exactly which of up to 64 physical ports this ACL entry's authorized source is permitted to arrive on, with each bit position independently controlling one specific port number, and an all-ones mask meaning the source is authorized regardless of which port it comes in on. An admin uses a restrictive port mask specifically to scope an authorization down to the expected physical connection point for that source, so that if the same source address were somehow injected or spoofed on a different, unexpected port, this entry's port check would reject it even though the source address itself matches. For example, an admin authorizing a partner's subnet that should only ever arrive via the dedicated link on port 12 would set this mask to permit only bit 11 (port 12), so any traffic claiming that same source address but arriving on a different port gets rejected as a likely spoofing attempt rather than silently passed.