What is the zone ID and why is it needed?  

When using site-local addresses, it is possible to be connected to multiple sites. In this case, each site is assigned a site identifier. To define the area of the network for which the destination is intended, the zone ID is used to indicate the site identifier. In the IPv6 protocol, the zone ID is the Zone ID for Site from the display of the netsh interface ipv6 show interface level=verbose command. If you are connected only to a single site, the default site identifier is 1 and the zone ID does not have to be specified. The site identifier is defined locally on each IPv6 host. Because of this, the site identifier used by Host A to reach Host B might not be the same as the site identifier used by Host B to reach Host A. You can determine your Zone ID number by opening a command prompt window and entering the command ipconfig. This command returns information specific to your local system’s addresses.

The notation that is used to specify the zone ID with an address is Address%ZoneID.

Answered by Karthick AR at 12:05 PM on November 03, 2008

In the IPv6 protocol, the zone ID for link-local addresses is typically the interface index from the display of the netsh interface ipv6 show interface command. The interface index is defined locally on each IPv6 host. Because of this, the interface index used by Host A to reach Host B might not be the same as the interface index used by Host B to reach Host A.

When using site-local addresses, it is possible to be connected to multiple sites. In this case, each site is assigned a site identifier. To define the area of the network for which the destination is intended, the zone ID is used to indicate the site identifier. In the IPv6 protocol, the zone ID is the Zone ID for Site from the display of the netsh interface ipv6 show interface level=verbose command. If you are connected only to a single site, the default site identifier is 1 and the zone ID does not have to be specified. The site identifier is defined locally on each IPv6 host. Because of this, the site identifier used by Host A to reach Host B might not be the same as the site identifier used by Host B to reach Host A.

For example, host A has two interfaces which automatically receive link-local addresses when activated (per RFC 2462): fe80::1/64 and fe80::2/64, only one of which is connected to the same physical network as host B which has address fe80::3/64, if host A attempts to contact fe80::3 how does it know which interface (fe80::1 or fe80::2) to use?

The solution defined by RFC 4007 is the addition of a unique zone index for the local interface, represented textually in the form <address>%<zone_id>, for example: http://[fe80::1122:33ff:fe11:2233%e th0]:80/ - this however may cause its own problems due to clashing with the percent-encoding used with URIs. [1]

Microsoft Windows IPv6 stack uses numeric zone IDs: fe80::3%1
BSD applications typically use the interface name as a zone ID: fe80::3%pcn0
Linux applications also typically use the interface name as a zone ID: fe80::3%eth0, although Linux ifconfig as of version 1.42 (part of net-tools 1.60) does not display zone IDs.
Relatively few IPv6-capable applications understand zone ID syntax (with the notable exception of OpenSSH), thus rendering link-local addresses unusable within them if multiple interfaces use link-local addresses.

Answered by Nagendra at 8:05 AM on November 03, 2008

Link-local and site-local addresses can be reused. Link-local addresses are reused on each link. Site-local addresses can be reused within each site of an organization. Because of this address reuse capability, link-local and site-local addresses are ambiguous. To specify which link on which the destination is located or within which site the destination is located, an additional identifier is needed. This additional identifier is a zone identifier (ID), also known as a scope ID, which identifies a connected portion of a network that has a specified scope.

The syntax specified in RFC 4007 for identifying the zone associated with a link-local or site-local address is Address%zone_ID in which Address is a link-local or site-local unicast IPv6 address and zone_ID is an integer value representing the zone. The values of the zone ID are defined relative to the sending host. Therefore, different hosts might determine different zone ID values for the same physical zone. For example, Host A might choose 3 to represent the zone of an attached link and host B might choose 4 to represent the same link.

Answered by Anju Singh at 7:14 AM on November 03, 2008