code: func (m *Mapping) validGateway(ip net.IP) bool {
  	if m.address.IP == nil || ip == nil || m.address.IP.IsUnspecified() || ip.IsUnspecified() {
  		return true
  	}
  	return m.address.IP.Equal(ip)
  }
docs: // Checks if the mappings local IP address matches the IP address of the gateway
  // For example, if we are explicitly listening on 192.168.0.12, there is no
  // point trying to acquire a mapping on a gateway to which the local IP is
  // 10.0.0.1. Fallback to true if any of the IPs is not there.