code: func (s *Stack) AddSubnet(id tcpip.NICID, protocol tcpip.NetworkProtocolNumber, subnet tcpip.Subnet) *tcpip.Error {
  	s.mu.RLock()
  	defer s.mu.RUnlock()
  
  	if nic, ok := s.nics[id]; ok {
  		nic.AddSubnet(protocol, subnet)
  		return nil
  	}
  
  	return tcpip.ErrUnknownNICID
  }
docs: // AddSubnet adds a subnet range to the specified NIC.