code: func (n *network) getSubnetforIPv4(ip *net.IPNet) *ipv4Subnet {
  	for _, s := range n.config.Ipv4Subnets {
  		_, snet, err := net.ParseCIDR(s.SubnetIP)
  		if err != nil {
  			return nil
  		}
  		// first check if the mask lengths are the same
  		i, _ := snet.Mask.Size()
  		j, _ := ip.Mask.Size()
  		if i != j {
  			continue
  		}
  		if snet.Contains(ip.IP) {
  			return s
  		}
  	}
  
  	return nil
  }
docs: // getSubnetforIPv4 returns the ipv4 subnet to which the given IP belongs