In programming APIs (not in communication between hosts), requests a system-allocated (dynamic) port
1
Yes
Assigned
Official
TCP Port Service Multiplexer (TCPMUX). Historic. Both TCP and UDP have been assigned to TCPMUX by IANA, but by design only TCP is specified.
5
Assigned
Assigned
Official
Remote Job Entry was historically using socket 5 in its old socket form, while MIB PIM has identified it as TCP/5 and IANA has assigned both TCP and UDP 5 to it.