Network Identifiers Cheat Sheet

Practical Snippet

Key points: (1) IPv6 address classes are determined by the leading bits — fc00::/7 unique local, fe80::/10 link-local, ff for multicast, 2000::/3 global unicast; (2) fe80::/10 link-local addresses are valid only on the same L2 link and are never forwarded, so writing one into a config as a fixed address fails on a different network, and tests need a zone index such as %eth0; (3) a form like ::ffff:192.168.1.1 is an IPv4-mapped address used for compatibility rather than real IPv6, and dual-stack applications must recognise it; (4) link-local multicast such as ff02::/16 also needs a zone index, and omitting it is the most common mDNS/SSDP mistake; (5) IPv6 typically allocates a fixed /64 per user, and that granularity determines room for later subnetting.

// ① 按前导位判断 IPv6 地址类别
function classifyIPv6(addr) {
  const a = addr.toLowerCase().split('%')[0]           // 去掉 zone index
  if (a === '::1') return { kind: 'loopback', note: '本机环回' }
  if (a === '::') return { kind: 'unspecified', note: '未指定' }
  if (a.startsWith('::ffff:')) {
    // 🔴 IPv4 映射地址：兼容用，不是真正的 IPv6 通信
    const v4 = a.slice(7)
    const isMapped = /^d{1,3}(.d{1,3}){3}$/.test(v4)
    return { kind: isMapped ? 'ipv4-mapped' : 'nat64', v4, note: 'IPv4 映射/兼容地址' }
  }
  const first = parseInt(a[0], 16)                      // 首个十六进制位
  if ((first & 0xfe) === 0xfc) return { kind: 'unique-local', cidr: 'fc00::/7', note: '唯一本地' }
  if (a.startsWith('fe8') || a.startsWith('fe9') || a.startsWith('fea') || a.startsWith('feb')) {
    return { kind: 'link-local', cidr: 'fe80::/10', note: '链路本地（仅同链路，跨网段不通）' }
  }
  if (first === 0xff) {
    const second = parseInt(a.split(':')[1] || '0', 16)
    const scope = second === 0x01 ? 'node' : second === 0x02 ? 'link' : second === 0x05 ? 'site' : 'global'
    return { kind: 'multicast', scope, note: '组播（链路本地需 zone index）' }
  }
  if (a.startsWith('200')) return { kind: 'global-unicast', cidr: '2000::/3', note: '全球单播' }
  if (a.startsWith('64:ff9b')) return { kind: 'nat64', cidr: '64:ff9b::/96', note: 'NAT64' }
  return { kind: 'other', first: first.toString(16) }
}

console.log(classifyIPv6('::1'))              // loopback
console.log(classifyIPv6('2001:db8::1'))      // global-unicast
console.log(classifyIPv6('fe80::1'))          // link-local
console.log(classifyIPv6('fc00::1'))          // unique-local
console.log(classifyIPv6('ff02::1'))          // multicast / link
console.log(classifyIPv6('::ffff:192.168.1.1'))  // ipv4-mapped

// ② 链路本地与组播必须带 zone index（网卡名）
// ✅ ping6 fe80::1%eth0
// ❌ ping6 fe80::1        —— 多网卡时无法确定从哪块发出

// ③ 分配粒度：IPv6 通常固定 /64 给用户
//   /64 = 2^64 个地址 ⇒ 不需要再分子网给设备
//   /128 = 单个接口（loopback 用）
//   🔴 分配粒度决定后续扩展空间，错了无法回收