fastping.go
func (p *Pinger) AddIP(ipaddr string) error {
	addr := net.ParseIP(ipaddr)
	if addr == nil {
		return fmt.Errorf("%s is not a valid textual representation of an IP address", ipaddr)
	}
	p.mu.Lock()
	p.addrs[addr.String()] = &net.IPAddr{IP: addr}
	if isIPv4(addr) {
		p.hasIPv4 = true
	} else if isIPv6(addr) {
		p.hasIPv6 = true
	}
	p.mu.Unlock()
	return nil
}
func (p *Pinger) AddIP(ipaddr string) error {
	addr := net.ParseIP(ipaddr)
	if addr == nil {
		return fmt.Errorf("%s is not a valid textual representation of an IP address", ipaddr)
	}
	p.mu.Lock()
	p.addrs[addr.String()] = &net.IPAddr{IP: addr}
	if isIPv4(addr) {
		p.hasIPv4 = true
	} else if isIPv6(addr) {
		p.hasIPv6 = true
	}
	p.mu.Unlock()
	return nil
}
// AddIP adds an IP address to Pinger. ipaddr arg should be a string like
// "192.0.2.1".