Cisco CCNA Study Guide - Router Alley

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CCNA Study Guide v2.62 – Aaron Balchunas 106 The IPv6 Prefix IPv4 utilizes a subnet mask to define the network “prefix” and “host” portions of an address. This subnet mask can also be represented in Classless Inter-Domain Routing (CIDR) format. IPv6 always use CIDR notation to determine what bits notate the prefix of an address: Full Address: 1254:1532:26B1:CC14:123:1111:2222:3333/64 Prefix ID: 1254:1532:26B1:CC14: Host ID: 123:1111:2222:3333 The /64 indicates that the first 64 bits of this address identify the prefix. The IPv6 Interface ID and EUI-64 Format The host portion of an IPv4 address is not based on the hardware address of an interface. IPv4 relies on Address Resolution Protocol (ARP) to map between the logical IP address and the 48-bit hardware MAC address. IPv6 unicasts generally allocate the first 64 bits of the address to identify the network (prefix), and the last 64 bits to identify the host (referred to as the interface ID). The interface ID is based on the interface’s hardware address. This interface ID adheres to the IEEE 64-bit Extended Unique Identifier (EUI-64) format. Since most interfaces still use the 48-bit MAC address, the MAC must be converted into the EUI-64 format. Consider the following MAC address: 1111.2222.3333. The first 24 bits, the Organizationally Unique Identifier (OUI), identify the manufacturer. The last 24 bits uniquely identify the host. To convert this to EUI-64 format: 1. The first 24 bits of the MAC (the OUI), become the first 24 bits of the EUI-64 formatted interface ID. 2. The seventh bit of the OUI is changed from a “0” to a “1”. 3. The next 16 bits of the interface ID are FFFE. 4. The last 24 bits of the MAC (the host ID), become the last 24 bits of the interface ID. Thus, the MAC address 1111.2222.3333 in EUI-64 format would become 1311:22FF:FE22:3333, which becomes the interface ID. * * * All original material copyright © 2013 by Aaron Balchunas (aaron@routeralley.com), unless otherwise noted. All other material copyright © of their respective owners. This material may be copied and used freely, but may not be altered or sold without the expressed written consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
CCNA Study Guide v2.62 – Aaron Balchunas 107 The IPv6 Address Hierarchy IPv4 separated its address space into specific classes. The class of an IPv4 address was identified by the high-order bits of the first octet: • Class A - (00000001 – 01111111, or 1 - 127) • Class B - (10000000 – 10111111, or 128 - 191) • Class C - (11000000 – 11011111, or 192 - 223) • Class D - (11100000 – 11101111, or 224 - 239) IPv6’s addressing structure is far more scalable. Less than 20% of the IPv6 address space has been designated for use, currently. The potential for growth is enormous. The address space that has been allocated is organized into several types, determined by the high-order bits of the first field: • Special Addresses – addresses begin 00xx: • Link Local – addresses begin FE8x: • Site Local – addresses begin FECx: • Aggregate Global – addresses begin 2xxx: or 3xxx: • Multicasts – addresses begin FFxx: • Anycasts (Note: an “x” indicates the value can be any hexadecimal number) There are no broadcast addresses in IPv6. Thus, any IPv6 address that is not a multicast is a unicast address. Anycast addresses identify a group of interfaces on multiple hosts. Thus, multiple hosts are configured with an identical address. Packets sent to an anycast address are sent to the nearest (i.e., least amount of hops) host. Anycasts are indistinguishable from any other IPv6 unicast address. Practical applications of anycast addressing are a bit murky. One possible application would be a server farm providing an identical service or function, in which case anycast addressing would allow clients to connect to the nearest server. * * * All original material copyright © 2013 by Aaron Balchunas (aaron@routeralley.com), unless otherwise noted. All other material copyright © of their respective owners. This material may be copied and used freely, but may not be altered or sold without the expressed written consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
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