| IPv6 Network - IP Version 6 | | Print | |
Last but not least, there is IPv6. There are many differences between IPv4 and IPv6, but it’s important to recognize that IPv6 is still IP. Any and all protocols that run over IPv4 can also run over IPv6, assuming the necessary changes are made to accommodate the larger addresses. IPv6 addresses are 128 bits (16 bytes) long and fully classless. Well, in theory at least. In practice, in almost all cases, 64 bits are used to number networks and the remaining 64 bits are host bits. The 64 host bits are by default filled in with an Extended Unique Identifier (EUI-64), which is simply a 64-bit MAC address.
Regular 48-bit Ethernet MAC addresses can easily be turned into EUI-64s by filling up the missing bits in the middle with 15 ones and a zero. The network part of the address is filled in with a network address that is periodically broadcast by routers. So it would seem that IPv6 adopts the IPX/CLNP approach by including the MAC address in the network layer address. However, this isn’t the whole story. IPv6 also borrows from AppleTalk by performing a Duplicate Address Detection (DAD) procedure over the address, which means the actual address used may not contain a valid EUI-64. Just to be sure,
IPv6 also supports the traditional IPv4 ways to assign addresses: through manual configuration and using an IPv6 version of the Dynamic Host Configuration Protocol (DHCP).
Because the host bits in the IPv6 address don’t necessarily contain a MAC address, IPv6 includes an ARP-like mechanism to discover MAC addresses. But ARP is a fairly Ethernet-specific protocol, and it uses broadcasts, which IPv6 doesn’t support. Instead, IPv6 uses multicasts extensively. Multicasts are like targeted broadcasts: packets are delivered to all hosts that are subscribed to a certain multicast group address. There are different group addresses for different purposes. Neighbor Discovery (ND), which is IPv6’s replacement for ARP, is entirely multicast-based and is also more generic than ARP, removing the Ethernet-centrism and supporting additional capabilities such as dead neighbor detection. Figure 1-7 shows the relationship between the Ethernet MAC address, the EUI-64, and the IPv6 address.
This short overview may give you the impression that IPv6 is unnecessarily complex, but in my opinion, that’s not the case. Yes, of the six network layer protocols, IPv6 is the most complex (or maybe it’s a tie with CLNS), but as you read the rest of the book, you’ll find out that IPv6 takes the best features from its direct and less direct predecessors, adds a few new ones of its own, and melds them into something elegant and powerful.
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