Working on regtest (Bitcoin Core 0.15.1) I've realized that some peers are identified just by ip instead of by ip:port when querying bitcoin-cli getpeerinfo. I've noticed that when it happens the connection is always outbound, so the port is actually the one set in the node (in this case 18444), but it doesn't happens with all outbound connections. Does anyone have a clue about what could be the reason?

e.g:

{ "id": 41, "addr": "172.192.0.5", "addrlocal": "172.192.0.2:56402", "addrbind": "172.192.0.2:56402", "services": "000000000000000d", "relaytxes": true, "lastsend": 1516808394, "lastrecv": 1516808394, "bytessent": 17600, "bytesrecv": 93216, "conntime": 1516786190, "timeoffset": 0, "pingtime": 0.000588, "minping": 0.000101, "version": 70015, "subver": "/Satoshi:0.15.1/", "inbound": false, "addnode": true, "startingheight": 1, "banscore": 0, "synced_headers": 101, "synced_blocks": 101, "inflight": [ ], ... }

Just in case it could matter, all nodes are docker containers connected to the same virtual network (all hosted by the same computer).