It uses the port you tell it to. -o OR -x options are relevant depending on your proxy situation or not.  You have to use ports specified by the mining server as available for use, they often vary greatly depending on the pool.

minerd --help
Usage: minerd [OPTIONS]
Options:
  -a, --algo=ALGO       specify the algorithm to use
                          scrypt    scrypt(1024, 1, 1) (default)
                          sha256d   SHA-256d
                          X11       Xcoin
  -o, --url=URL         URL of mining server (default: http://127.0.0.1:9332/)
  -O, --userpass=U:P    username:password pair for mining server
  -u, --user=USERNAME   username for mining server
  -p, --pass=PASSWORD   password for mining server
      --cert=FILE       certificate for mining server using SSL
  -x, --proxy=[PROTOCOL://]HOST[:PORT]  connect through a proxy
  -t, --threads=N       number of miner threads (default: number of processors)
  -r, --retries=N       number of times to retry if a network call fails
                          (default: retry indefinitely)
  -R, --retry-pause=N   time to pause between retries, in seconds (default: 30)
  -T, --timeout=N       network timeout, in seconds (default: 270)
  -s, --scantime=N      upper bound on time spent scanning current work when
                          long polling is unavailable, in seconds (default: 5)
      --no-longpoll     disable X-Long-Polling support
      --no-stratum      disable X-Stratum support
  -q, --quiet           disable per-thread hashmeter output
  -D, --debug           enable debug output
  -H, --hashdebug       enable hash debug output
  -P, --protocol-dump   verbose dump of protocol-level activities
  -S, --syslog          use system log for output messages
  -B, --background      run the miner in the background
      --benchmark       run in offline benchmark mode
  -c, --config=FILE     load a JSON-format configuration file
  -V, --version         display version information and exit
  -h, --help            display this help text and exit