code: func (n *NIC) AddSubnet(protocol tcpip.NetworkProtocolNumber, subnet tcpip.Subnet) {
  	n.mu.Lock()
  	n.subnets = append(n.subnets, subnet)
  	n.mu.Unlock()
  }
docs: // AddSubnet adds a new subnet to n, so that it starts accepting packets
  // targeted at the given address and network protocol.