After creating a replica set with config where hosts are specified by name:port getLastError replicatedTo field lists only ip addresses and no ports.

rs.conf()
...
members: [
...
{
"_id":1,
"host": "asyasmacbook.local:31001",
etc.

rs.status()
...
members: [
...
"name": "asyasmacbook.local:31001",
etc.

getlasterror returns:
...
"replicatedTo" : [
"127.0.0.1",
"127.0.0.1"
], etc.

Since people may be running multiple servers on the same machine port name seems absolutely necessary. I'm not not sure why hostname isn't returned in the same form it's displayed in config and status but it seems potentially confusing.