code: func (p *Prefix) Last() net.IP {
  	if p.IP.To4() != nil {
  		i := p.lastIPv4Int()
  		return i.ip()
  	}
  	if p.IP.To16() != nil && p.IP.To4() == nil {
  		i := p.lastIPv6Int()
  		return i.ip()
  	}
  	return nil
  }
docs: // Last returns the last IP in the address range of p.
  // It returns the address of p when p contains only one address.