Network Identifiers Cheat Sheet
MAC addresses, IPv4 private ranges, IPv6 global unicast, ULA and link-local all differ in bit width, notation and scope — and confusing them is one of the most common sources of network misconfiguration. This page puts them side by side and lists the IPv6 prefix types you will actually meet.
Five Identifier Types
| Identifier | Bits | Example | Scope | Assigned By |
|---|---|---|---|---|
| MAC 地址 | 48 | 00:1A:2B:3C:4D:5E | Unique within a single link (layer 2) | Burned in by the NIC vendor; first 24 bits are the OUI |
| IPv4 私有地址 | 32 | 192.168.1.10 | Inside a LAN; requires NAT to reach the internet | Assigned by an administrator or DHCP |
| IPv6 全球单播 | 128 | 2001:db8::1 | Globally routable public address | Assigned by the ISP or an RIR |
| IPv6 唯一本地（ULA） | 128 | fd12:3456::1 | Site-internal; must not be routed on the internet | Generated with a random 40-bit global ID |
| IPv6 链路本地 | 128 | fe80::1 | A single link only; never forwarded by a router | Auto-configured when IPv6 is enabled on the interface |
Common IPv6 Prefix Types
| Prefix | Type | Description |
|---|---|---|
| ::1/128 | Loopback | The host itself, equivalent to IPv4 127.0.0.1 |
| 2000::/3 | Global Unicast | Publicly routable addresses; current allocations fall here |
| fc00::/7 | Unique Local | Site-internal; only those starting with fd are locally generated |
| fe80::/10 | Link-Local | Confined to one link; routers do not forward it |
| ff00::/8 | Multicast | One-to-many delivery; replaces IPv4 broadcast |
| 2002::/16 | 6to4 | Transition mechanism, now largely deprecated |
Frequently Asked Questions
Can a MAC address be used to locate a specific device?
Not across the public internet. A MAC address is only meaningful within a single layer-2 link; once a packet crosses a router the source MAC is rewritten, so outsiders never see your real MAC. All you can do is look up the vendor from the first 24 bits, the OUI — a prefix may belong to a particular phone or router maker. What actually tracks devices across networks are IP addresses, browser fingerprints and account systems. Modern operating systems also randomise MAC addresses when probing Wi-Fi, further weakening MAC-based tracking.
Are IPv6 ULAs the same thing as IPv4 private addresses?
The intent is similar but the details differ considerably. Both are for internal use and neither should be routed publicly. IPv4, however, offers only three fixed blocks — 10/8, 172.16/12 and 192.168/16 — so different organisations inevitably reuse the same addresses and mergers cause conflicts. ULA reserves a 40-bit global ID after the fc00::/7 prefix, and the standard requires it to be generated randomly, so collisions between organisations are vanishingly unlikely and merging networks rarely requires renumbering. In practice many networks run ULA alongside global unicast: ULA keeps internal communication always reachable, global unicast handles the outside world.
Why do IPv6 addresses start with fd rather than fc?
The fc00::/7 prefix is effectively split in two: fc00::/8 is reserved for future central allocation, while fd00::/8 is for locally generated addresses. RFC 4193 requires locally generated ULAs to set the eighth bit, the L bit, to 1, which means the prefix must begin with fd. Addresses produced by a local tool therefore always start with fd; an fc prefix is either reserved or a misconfiguration.
Why does IPv6 have no broadcast address?
IPv6 dropped broadcast in favour of multicast and anycast. Broadcast disturbs every host on the segment, and each one must interrupt what it is doing to process the packet, which is wasteful on large networks. Multicast is delivered only to hosts that joined the relevant group, leaving everyone else untouched. Address resolution, which relied on broadcast in IPv4, is handled in IPv6 by the Neighbour Discovery Protocol using ICMPv6 multicast. That is why you will never see an equivalent of 255.255.255.255 in IPv6.