The Internet has been growing exponentially since 1990, as more and more organizations enter cyberspace to facilitate business, research, and education. The downside to this phenomenal success is that the Internet faces a serious shortage of IP addresses, those unique strings of binary numbers that identify Internet hosts. In the early 1990s, people predicted that the last Class B IP address would be allocated in March 1994, a month dubbed the Date of Doom. Although researchers developed interim solutions to postpone the Date of Doom, today it's happening all over again: All current IP addresses will be depleted sometime between 2005 and 2010, if the current rate of Internet growth continues.
Fortunately, the Internet Engineering Task Force (IETF), the organization that developed protocol standards for the Internet, foresaw the diminishing IP address problem and other problems related to IP version 4 (IPv4). To address these problems, the IETF developed IP next generation (IPng), and in January 1995 published "The Recommendation for the IP Next Generation Protocol" in its Request for Comments (RFC) 1752. The IETF referred to the new-generation IP as IP version 6 (IPv6) and developed a comprehensive set of IPv6 standards specifying the implementation of IPv6 on the Internet. In addition to its 128-bit address space, which will solve the address-exhaustion problem, IPv6 uses a hierarchical address scheme, an efficient IP header, Quality of Service (QoS), host address autoconfiguration, authentication, and encryption. Because IPv6 differs in important ways from IPv4, the IETF also created a transition mechanism to ease migration from IPv4 to IPv6.
Vendors have started to support IPv6 and deliver IPv6 products. For example, FTP Software has delivered IPv6 stacks for Windows NT 4.0 and Windows 95. IPv6 has already been built into many routers and into UNIX. The Internet backbone for IPv6 testing, 6bone, links 29 countries to develop IPv6 technologies. IPv6 will eventually arrive in your organization. Be sure you understand IPv6 so that you can apply this new technology properly in your network. In this article I'll delve into IPv6 and explain its address scheme, address autoconfiguration, security, and transition mechanism. As you read, you'll see the benefits IPv6 can bring to the Internet and your intranet.
The IPv4 Address Problem
The Internet originated in the Advanced Research Projects Agency Network (ARPANET), which connected government contractors for the US Depart-
ment of Defense (DoD). The research for ARPANET began in 1968, and researchers developed IP to standardize communication protocols in ARPANET. Its developers assumed ARPANET would have fewer than several dozen networks. They selected an address-space size of 32 bits: The first 8 bits represented the network (8 bits can identify 28, or 256 networks), and the remaining 24 bits represented the host. As ARPANET grew, its developers realized it would have more than 256 networks, so they separated the 32-bit address space into three classes: Class A, for large networks; Class B, for midsized networks; and Class C, for small networks.
IPv4's Class A 32-bit addresses begin with a 0 bit, followed by a 7-bit identifier and a 24-bit host identifier. Thus, Class A addresses can identify 27, or 128, networks, each of which can have at most 224, or 16,777,216, hosts. Class B 32-bit addresses begin with the bits 1 0, followed by a 14-bit network identifier and a 16-bit host identifier. Class B addresses can identify 214, or 16,384, networks, each with at most 216, or 65,536, hosts. Class C 32-bit addresses begin with the bits 1 1 0, followed by a 21-bit network identifier and an 8-bit host identifier. Class C addresses can identify 221, or 2,097,152, networks, each with at most 28, or 256, hosts. As you can see, there's a big difference between the number of hosts Class B addresses can handle compared with Class C addresses. Organizations that had or expected to have more than 256 hosts needed a Class B address. By 1992, the InterNIC had assigned about half of the available Class B addresses, and industry analysts projected the Date of Doom from the existing address-assignment rates.
Classless interdomain routing (CIDR), an immediate solution to the Date of Doom, came to the rescue. The idea behind CIDR is to give a block of contiguous Class C addresses, rather than a Class B address, to a company that has more than 256 but fewer than several thousand hosts. For example, suppose you have 1500 hosts on your network. You might receive eight contiguous Class C addresses, such as 192.56.0.0 to 192.56.7.0, with a subnet mask (a pattern of bits that establishes which part of the IP address identifies the network and which part identifies the host) of 255.255.248.0. All the addresses share the most significant (the higher order) 21 bits, followed by 11 bits to identify up to 2048 hosts. By using Class C addresses in this way, CIDR saved Class B addresses from depletion. Unfortunately, CIDR has not solved the IPv4 problem--the InterNIC will allocate all IPv4 addresses one day, and that day will come within the next 10 to 12 years, according to current projections.
Another mechanism is delaying IPv4 address exhaustion: network address translation. NAT was born from fire-wall technology, in which a company enhances its network security by hiding its internal IP addresses from the external network. Using NAT, a company doesn't need globally unique or legitimate addresses for its private network. When NAT sits on the border between a company's network and the Internet, NAT can convert the company's private IP address space to a small pool of globally unique addresses. Because acquiring a Class A or B address is difficult, many large companies use the private addresses that NAT creates for their internal networks.
However, NAT degrades performance in network throughput. NAT must convert addresses for all packets passing to or from the Internet, but most NATs can't pass this address information to the packet payload (contents). This inability leads to application failures when higher-layer (above the network layer) applications, such as FTP and Windows Internet Naming Service (WINS) registration, must embed address information in a packet's payload.
The IPv6 Address Answer
IPv6 overcomes the address-space problem in IPv4 by defining a 128-bit address space. This address space is long enough to connect all of a company's equipment (e.g., computers, printers, pagers) to the Internet without address conflicts.
IPv6 expresses addresses differently than IPv4 does. An IPv6 address contains eight sections separated by colons. Each section contains 16 bits expressed in four hexadecimal numbers. An example IPv6 address is 1234:5678:9ABC:DEF0:1234:
5678:9ABC:DEF0.
Memorizing an IPv6 address isn't easy. Fortunately, IPv6 lets you simplify an address by cutting off the leading zeros from any 16-bit section that contains them and using a double colon (::) to indicate multiple contiguous sections of zeros. For example, you can simplify address 0123:0000:0000:0000:0004:0056:
789A:BCDE to 123::4:56:789A:BCDE. You can use only one double colon in a simplified address; otherwise, IPv6 could not calculate how many 16-bit sections of zeros occur in a simplified address.
In addition to its 128-bit address space, IPv6 designates a hierarchical address for point-to-point communication. IPv6 calls this type of address an aggregatable global unicast address. IPv6 partitions this address into the hierarchical format shown in Figure 1. The number at the beginning of the address is a format prefix that differentiates the aggregatable global unicast address from other types of addresses. At the top of the address hierarchy are top level aggregators (TLAs). TLAs are public network access points (NAPs) that interconnect long-distance service providers and telephone companies. International Internet registries, such as Internet Assigned Numbers Authority (IANA) allocate addresses to TLAs.
In turn, TLAs assign addresses to the next level in the aggregatable global unicast address hierarchy, the next level aggregator (NLA). NLAs are large Internet Service Providers (ISPs). An NLA allocates addresses to the next level in the aggregatable global unicast address hierarchy, the site level aggregator (SLA). An SLA, which is often called a subscriber, can be an organization such as a university or a small ISP. SLAs can assign addresses to their subscribers. In general, SLAs provide subscribers with a block of contiguous addresses so that organizations can create their address hierarchy to identify different subnets.
The last level of the aggregatable global unicast address is the host interface ID, which identifies one host interface. Organizations assign host interface IDs by using a unique number on the subnet, or they can use the host's NIC ID (i.e., the media access control--MAC--address).
Currently, the routing table of an Internet backbone router contains tens of thousands of entries that it uses to look up the path to a destination network. Routing tables keep growing, but a large routing table degrades a router's performance and can cause routing instabilities. The design of the aggregatable global unicast address can reduce a routing table's size by route aggregation or summarization. For example, with aggregatable global unicast addressing, a US backbone router needs only one entry (i.e., TLA) in its routing table for all networks in the UK. When the router receives a packet addressed to a network in the UK, it uses the TLA ID in the packet's destination address to find the path to the UK TLA in its routing table; then the router forwards the packet to the UK TLA. The UK TLA examines the NLA ID in the packet's destination address to determine the routing path to the NLA and sends the packet to the NLA. Finally, the NLA delivers the packet to its destination network according to the SLA ID in the destination address. This efficient global routing hierarchy operates similarly to the public telephone network.
Three Types of IPv6 Addresses
The aggregatable global unicast address is only a part of IPv6 address space. IPv6 defines three types of addresses: unicast, multicast, and anycast. Unicast traffic is the most common traffic on the Internet (a unicast address specifies one recipient). The aggregatable global unicast address is well designed for this point-to-point communication. IPv6 also defines two special unicast addresses for intranets. The first is the link local unicast address, and the second is the site local unicast address. You use the link local unicast address if you let packets traverse on only one link or segment. Routers will not forward packets with link local unicast addresses. You use the site local unicast address if you want to limit the packet delivery scope to your intranet. The edge router connecting your internal network to the external network will never forward packets with site local unicast addresses to the external network.
As in IPv4, IPv6 multicast addresses deliver copies of one source packet to recipients. All recipient hosts in the multicast group receive copies of the same message from one multicast stream. IPv6 supports two kinds of multicast addresses: permanent and transient. Permanent multicast addresses are well-known multicast addresses for special uses, such as for all routers in a local network. You can define a transient multicast address for a multicast group in your network, such as an audio conference. The IPv6 multicast address contains a 112-bit multicast group ID. This address lets you designate a large number of multicast groups for your multicast applications. In the IPv6 multicast address, you can specify multicast scope, which can be node-local, link-local, site-local, or global. In IPv6, multicasting to all nodes in your organization replaces the broadcasting capability in IPv4.
IPv6 introduces a third type of address, the anycast address. Anycast addresses deliver a message to a group of nodes. You use an anycast address to represent a group of nodes. Anycast differs from multicast in that it delivers a message to any one of the nodes in a group. When one node, often the nearest node in the group, receives the message, anycast is finished. You can group routers in an anycast group, and a host can send a query to the anycast group to find the nearest router. You can apply the same concept to other network systems or services, such as Domain Name System (DNS) servers. Currently, IPv6 limits anycast group members to routers only.
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