When you plan to nmap scan a network which contains printers, avoid scanning the JetDirect port, port 9100.

Newer versions of nmap skips TCP port 9100 because some printers simply print anything sent to that port, leading to dozens of pages of HTTP get requests, binary SSL session requests, etc. This behavior can be changed by modifying or removing the Exclude directive in nmap-service-probes, or you can specify –allports to scan all ports regardless of any Exclude directive.