Authorized reprint by author zhushigeng (Vinsoney)
Unicast Address
1. Aggregatable Global Unicast Address
· An aggregatable global unicast address is also called a global unicast address, which is globally unique and is equivalent to an IPv4 public address.
· The prefix 2000::/3 is used. Generally, the IPv6 address space obtained from a carrier is /48. You can plan the IPv6 address space as required.
· An IPv6 address allocated to the terminal may have the structure as shown in the following figure.
· Global route prefix: is specified by a provider to an organization. Generally, the value is at least 48 bits (including the fixed three bits). Currently, the first three bits of every assigned global routing prefix are 001. Therefore, the prefix is 2000::/3.
· Subnet ID: is used by an organization to construct a local network.
· Interface ID: identifies a device (interface).
2. Site-Local Address
A site-local address in IPv6 is similar to a private IP address in IPv4. This type of address is prefixed with FEC0::/10. That is, the highest 10 bits are constantly 1111111011, followed by consecutive 0s of 38 bits. Therefore, a site-local address has fixed first 48 bits. Between the interface ID and the fixed 48 most significant bits lies a 16-bit subnet ID, which is used by an organization for constructing a subnet. A site-local address can be used only in the local or private environment and cannot be used to access the public network.
RFC 1884 defines that the FEC0::/10 address block is used for site-local addresses. These addresses can be used only in private IPv6 sites. However, due to historical reasons, the use of site-local addresses is out of order. As a result, these addresses are no longer used.
3. Unique Local Address
A unique local address is equivalent to a private address and can be used only on the local network. It cannot be routed on the IPv6 Internet. Site-local addresses are no longer used because they are not clearly defined. RFC redefines the unique local address so that private IPv6 addresses can be used in the local environment.
In RFC 4193, a type of address is standardized to replace unicast local addresses of a site in local communication. A unique local address has a fixed prefix, FC00::/7, which is divided into two blocks, FC00::/8 and FD00::/8. FC00::/8 is not defined yet. The following figure shows the format of FD00::/8.
A unique local address is valid only within a limited range, but also has a globally unique prefix. (Strictly, it could be almost globally unique). A global ID is generated in pseudo-random mode. RFC 4193 provides some suggestions on the generation mode, for example, "Obtain the current time of day in 64-bi****P format [NTP]."
4. Link-Local Address
A link-local address is a very important type of address, and it is valid only on the link where the address is located. It is prefixed with FE80::/10, where the first 11 to 64 bits are 0s, followed by a 64-bit interface ID. The following figure shows the format.
A network adapter of an IPv6 host or an interface of a network device may have multiple aggregatable global unicast addresses, but these interfaces must have a unique link-local address within a link range. Due to the stability and link uniqueness, link-local addresses are used for IGP neighbor relationship establishment, automatic address configuration, neighbor discovery, and router discovery.
The data packets with the source or destination IPv6 address being a link-local address will not be forwarded out of the originating link. In other words, the valid range of a link-local address is the local link.
Each IPv6 interface must have a link-local address. Huawei devices support automatic generation and manual configuration of link-local addresses. On a Huawei router, after an interface obtains an IPv6 global unicast address, the device automatically configures a link-local address for the interface.
If no global unicast IPv6 address is configured on the interface, run the ipv6 address auto link-local command to enable the device to automatically generate a link-local address. On a firewall, router, and switch, the automatically generated link-local address is generated based on an EUI-64-compliant interface ID. If a global unicast IPv6 address is configured on an interface, you do not need to run the preceding command to enable the device to automatically generate a link-local address. Once an interface obtains an IPv6 global unicast address, the interface automatically generates a link-local address.
5. Special Address
· Unspecified address (all-0 address, ::/128, or 0:0:0:0:0:0/128)
The address is used as the source address of some packets, for example, neighbor solicitation (NS) packets sent during duplicate address detection or request packets sent by a client during DHCPv6 initialization.
· Loopback address (0:0:0:0:0:1/128 or: 1/128)
It is used for local loopback. Packets sent to ::/1 are actually sent to the local end for loopback tests of the local protocol stack.
· IPv4-compatible IPv6 address
In transition technologies, some IPv6 addresses that contain IPv4 addresses are used. To make IPv4 addresses more prominent, an IPv6 address with an embedded IPv4 address is defined. The embedded IPv4 address format is a special representation used in the transition mechanism. In this method, the part of the IPv6 address is in the hexadecimal format, and the part of the IPv4 address is in the decimal format. For example:
0:0:0:0:0:0:192.168.1.2 or ::192.168.1.2 (96 zeros)
IPv4-compatible addresses are used for transition mechanisms, such as automatic IPv4-compatible tunnels and NAT-PT. This type of address is almost no longer used.
