Thursday, January 27, 2005

IPv6 Part - B-2 : Network part, also known as prefix

Now lets take a look at the different types of prefixes (and therefore address types):

- Link local address type

These are special addresses which will only be valid on a link of an interface. Using this address as destination the packet would never pass through a router. It's used for link communications such as:

  • anyone else here on this link?
  • anyone here with a special address (e.g. looking for a router)?

They begin with ( where "x" is any hex character, normally "0")

fe8x: <- currently the only one in use.
An address with this prefix is found on each IPv6-enabled interface
after stateless auto-configuration (which is normally always the case).

- Site local address type

These are addresses similar to the RFC 1918 / Address Allocation for Private Internets in IPv4 today, with the added advantage that everyone who use this address type has the capability to use the given 16 bits for a maximum number of 65536 subnets. Comparable with the in IPv4 today.
Another advantage: because it's possible to assign more than one address to an interface with IPv6, you can also assign such a site local address in addition to a global one.
It begins with:
fecx: <- most commonly used.
fefx:(where "x" is any hex character, normally "0")

- Global address type "(Aggregatable) global unicast"

Today, there is one global address type defined (the first design, called "provider based," was thrown away some years ago RFC 1884 / IP Version 6 Addressing Architecture [obsolete], you will find some remains in older Linux kernel sources).
It begins with (x are hex characters)
Note: the prefix "aggregatable" is thrown away in current drafts.There are some further subtypes defined…

- Multicast addresses

Multicast addresses are used for related services.
They alway start with (xx is the scope value)
They are split into scopes and types:

1. Multicast scopes:

Multicast scope is a parameter to specify the maximum distance a multicast packet can travel from the sending entity.
Currently, the following regions (scopes) are defined:

  • ffx1: node-local, packets never leave the node.
  • ffx2: link-local, packets are never forwarded by routers, so they never leave the specified link.
  • ffx5: site-local, packets never leave the site.
  • ffx8: organization-local, packets never leave the organization (not so easy to implement, must be covered by routing protocol).
  • ffxe: global scope.
  • others are reserved.

2. Multicast types:

There are many types already defined/reserved (see RFC 2373 / IP Version 6 Addressing Architecture for details). Some examples are:

  • All Nodes Address: ID = 1h, addresses all hosts on the local node (ff01:0:0:0:0:0:0:1) or the connected link (ff02:0:0:0:0:0:0:1).
  • All Routers Address: ID = 2h, addresses all routers on the local node (ff01:0:0:0:0:0:0:2), on the connected link (ff02:0:0:0:0:0:0:2), or on the local site (ff05:0:0:0:0:0:0:2).

3. Solicited node link-local multicast address:

Special multicast address used as destination address in neighborhood discovery, because unlike in IPv4, ARP no longer exists in IPv6.
An example of this address looks like ff02::1:ff00:1234
Used prefix shows that this is a link-local multicast address. The suffix is generated from the destination address. In this example, a packet should be sent to address "fe80::1234", but the network stack doesn't know the current layer 2 MAC address. It replaces the upper 104 bits with "ff02:0:0:0:0:1:ff00::/104" and leaves the lower 24 bits untouched. This address is now used `on-link' to find the corresponding node which has to send a reply containing its layer 2 MAC address.

- Anycast addresses

Anycast addresses are special addresses and are used to cover things like nearest DNS server, nearest DHCP server, or similar dynamic groups. Addresses are taken out of the unicast address space (aggregatable global or site-local at the moment). The anycast mechanism (client view) will be handled by dynamic routing protocols.
Note: Anycast addresses cannot be used as source addresses, they are only used as destination addresses:

- Subnet-router anycast address:

A simple example for an anycast address is the subnet-router anycast address. Assuming that a node has the following global assigned IPv6 address:
3ffe:ffff:100:f101:210:a4ff:fee3:9566/64 <- Node's address The subnet-router anycast address will be created blanking the suffix (least significant 64 bits) completely:
3ffe:ffff:100:f101::/64 <- subnet-router anycast address.

1 comment:

hassan said...

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