Network layer

At the data link layer, things are fairly simple. Machines agree on the format of packets and a standard 48-bit host addressing scheme. However, the packet format and encoding vary with different physical layers: Ethernet has one set of characteristics, while an X.25-based satellite network has another. Because there are many physical networks, there should ideally be a standard interface scheme so that it isn't necessary to re-implement protocols on top of each physical network and its peculiar interfaces. This is where the network layer fits in. The higher-level protocols, such as TCP (at the transport layer), don't need to know any details about the physical network that is in use. As mentioned before, TCP runs over Ethernet, fiber optic network, or other media; the TCP protocols don't care about the physical connection because it is represented by a well-defined network layer interface.The network layer protocol of primary interest to NFS and NIS is the Internet Protocol, or IP. As its name implies, IP is responsible for getting packets between hosts on one or more networks. Its job is to make a best effort to get the data from point A to point B. IP makes no guarantees about getting all of the data to the destination, or the order in which the data arrives -- these details are left for higher-level protocols to worry about.On a local area network, IP has a fairly simple job, since it just moves packets from a higher-level protocol down to the data link layer. In a set of connected networks, however, IP is responsible for determining how to get data from its source to the correct destination network. The process of directing datagrams to another network is called routing; it is one of the primary functions of the IP protocol. Appendix A, "IP Packet Routing" contains a detailed description of how IP performs routing.

Datagrams and packets

IP deals with data in chunks called datagrams. The terms packet and datagram are often used interchangeably, although a packet is a data link-layer object and a datagram is network layer object. In many cases, particularly when using IP on Ethernet, a datagram and packet refer to the same chunk of data. There's no guarantee that the physical link layer can handle a packet of the network layer's size. As previously mentioned, the largest packet that can be handled by the physical link layer is called the Maximum Transmission Unit, or MTU, of the network media. If the medium's MTU is smaller than the network's packet size, then the network layer has to break large datagrams down into packet-sized chunks that the data link and physical layers can digest. This process is called fragmentation. The host receiving a fragmented datagram reassembles the pieces in the correct order. For example, an X.25 network may have an MTU as small as 128 bytes, so a 1518-byte IP datagram would have to be fragmented into many smaller network packets to be sent over the X.25 link. For the scope of this tutorial, we'll use packet to describe both the IP and the data link-layer objects, since NFS is most commonly run on Ethernet rather than over wide-area networks with smaller MTUs. However, the distinction will be made when necessary, such as when discussing NFS traffic over a wide area point-to-point link.

IP host addresses

The internet protocol identifies hosts with a number called an IP address or a host address. To avoid confusion with MAC addresses (which are machine or station addresses), the term IP address will be used to designate this kind of address. IP addresses come in two flavors: 32-bit IP Version 4 (IPv4) or 128 bit IPv6 address. We will talk about IPv6 addresses later in this chapter. For now, we will focus on IPv4 addresses. IPv4 addresses are written as four dot-separated decimal numbers between 0-255 (a dotted quad):IP addresses must be unique among all connected machines. Connected machines in this case are any hosts that you can get to over a network or connected set of networks, including your local area network, remote offices joined by the company's wide-area network, or even the entire Internet community. For a standalone system or a small office that is not connected (via an IP network) to the outside world, you can use the standard, private network addresses assigned such purposes. See "IPv4 address classes" later in this chapter. If your network is connected to the Internet, you have to get a range of IP addresses assigned to your machines through a central network administration authority, via your Internet Service Provider. If you are planning on joining the Internet in the future, you will need to obtain an address from your network service provider. This may be either an actual provider of Internet service, or your own organization, if it has addresses to hand out. We won't go into this further in this tutorial.The IP address uniqueness requirement differs from that for MAC addresses. IP addresses are unique only on connected networks, but machine MAC addresses are unique in the world, independent of any connectivity. Part of the reason for the difference in the uniqueness requirement is that IPv4 addresses are 32 bits, while MAC addresses are 48 bits, so mapping every possible MAC address into an IPv4 address requires some overlap. There are a variety of reasons why the IPv4 address is only 32 bits, while the MAC address is 48 bits, most of which are historical.Since the network and data link layers use different addressing schemes, some system is needed to convert or map the IP addresses to MAC addresses. Transport-layer services and user processes use IP addresses to identify hosts, but packets that go out on the network need MAC addresses. The Address Resolution Protocol (ARP) is used to convert the 32-bit IPv4 address of a host into its 48-bit MAC address. When a host wants to map an IP address to a MAC address, it broadcasts an ARP request on the network, asking for the host using the IP address to respond. The host that sees its own IP address in the request returns its MAC address to the sender. With a MAC address, the sending host can transmit a packet on the Ethernet and know that the receiving host will recognize it.A host can have more than one IP address. Usually this is because the host is connected to multiple physical network segments (requiring one network interface, such as an Ethernet controller, per segment), or because the host has multiple interfaces to the same physical network segment.

IPv4 address classes

Each IPv4 address has a network number and a host number. The host number identifies a particular machine on an organization's network. IP addresses are divided into classes that determine which parts of the address make up the network and host numbers, as demonstrated in Table 1-2.

Table 1-2. IPv4 address classes

Address Class and First Octet Value Network Number Octets Host Number Octets Address Form Number of Networks Number of Hosts per Network Maximum Number of Hosts per Class
Class A: 1-126 1 3 N.H.H.H 126 256 - 2 2,113,928,964
Class B: 128-191 2 2 N.N.H.H 16,384 256 - 2 1,073,709,056
Class C: 192-223 3 1 N.N.N.H 2,097,152 254 532,676,608
Class D: 224-239 N/A N/A M.M.M.M N/A N/A N/A
Class E: 240-255 N/A N/A R.R.R.R N/A N/A N/A

Each N represents part of the network number and each H is part of the address's host number. The 8-bit octet has 256 possible values, but 0 and 255 in the last host octet are reserved for forming broadcast addresses.Network numbers with first octet values of 240-254 are reserved for future use. The network numbers 0, 127, 255, 10, 172.16-172.31, and 192.168.0-192.168.255 are also reserved:

Note that there are only 126 class A network numbers, but well over two million class C network numbers. When the Internet was founded, it was almost impossible to get a class A network number, and few organizations (aside from entire networks or countries) had enough hosts to justify a class A address. Most companies and universities requested class B or class C addresses. A medium-sized company, with several hundred machines, could request several class C network numbers, putting up to 254 hosts on each network. Now that the Internet is much bigger, the rules for class A, B, and C network number assignment have changed, as explained in "Classless IP addressing".Class D addresses look similar to the other classes in that each address consists of 4 octets with a value no higher than 255 per octet. Unlike classes A, B, and C, a class D address does not have a network number and host number. Class D addresses are multicast addresses, which are used to send messages to more than one recipient host, whereas IP addresses in classes A, B, and C are unicast addresses destined for one recipient. Multicast on the Internet offers plenty of potential for efficient broadcast of information, such as bulk file transfers, audio and video, and stock pricing information, but has achieved limited deployment. There is an ongoing experiment known as the "MBONE" (Multicast backBONE) on the Internet to exploit this technology.Class E addresses are reserved for future assignment.

Classless IP addressing

In the early 1990s, due to the advent of the World Wide Web, the Internet's growth exploded. In theory, if you sum the maximum number of hosts per classes A, B, and C (refer back to Table 1-2), the Internet can have a potential for over 3.7 billion hosts. In reality, the Internet was running out of address capacity for two reasons.The first had to do with the inefficiencies built into the class partitioning. About 3.2 billion of the theoretical number of hosts were class A and class B, leaving about 500 million class C addresses. Most organizations did not need class A or class B addresses, and of those that did, a significant fraction of their assigned address space was not needed. Most users could get by with a class C network number, but the typical small business or home user did not need 254 hosts. Thus, the number of class C addresses was bounded by the maximum number of class C networks, about two million, which is far less than the number of users on the Internet.The problem of only two million class C networks was mitigated by the introduction of dynamically assigned IP addresses, and by the introduction of policies that tended to assign IP network numbers only to Internet Service Providers (ISPs), or to organizations that effectively acted as their own ISP, which would then use the free market to efficiently reallocate the IP addresses dynamically or statically to their customers. Thus most Intenet users get assigned a single IP address, and the ISP is assigned the corresponding network number.The second reason was routing scalability. When the Internet was orders of magnitude smaller then it is today, most address assignments were for class A or B and so routing between networks was straightforward. The routers simply looked at the network number, and sent it to a router responsible for that route. With the explosion of the Internet, and with most of that growth in class C network numbers, each network's router might have to maintain tables of hundreds of thousands of routes. As the Internet grew rapidly, keeping these tables up to date was difficult.This situation was not sustainable, and so the concept of "classless addressing" was introduced. With the exception of grandfathered address assignments, each IP address, regardless of whether it's class A, B, or C, would not have an implicit network number part and host number part. Instead the network part would be designated explicitly via a suffix of the form: "/XX", where XX is the number of bits of the IP address that refer to the network. Those organizations that needed more than the 254 hosts that a class C address would provide, would instead be assigned consecutive class C addresses. For example, an ISP that was assigned 192.1.2 and 192.1.3 could have a classless network number of 192.1.3.0/23. Any router on a network other than 192.1.2 or 192.1.3 that wanted to send to either network number would instead route to a single router associated with the classless network number 192.1.3.0/23 (i.e., any IP address that had its first 23 bits equal to 1100 0000 0000 0001 0000 001).With this new scheme, larger organizations get more consecutive class C network numbers. Within their local networks ("Intranets"), they can either use traditional class-based routing or classless routing that further subdivides the local network address space that can be used. The largest organizations may find that class-based routing doesn't scale, and so classless routing is the best approach.

Virtual interfaces

In "IP host addresses", we noted that a host could have multiple IP addresses assigned to it if it had multiple physical network interfaces. It is possible for a physical network segment to support more than one IP network number. For example, a segment might have 128.0.0.0/16 and 192.4.5.6/24. Some hosts on that segment might want to directly address hosts with either network number. Some operating systems, such as Solaris, will let you define multiple virtual or logical interfaces for a physical network interface. On most Unix systems, the ifconfig command is used to set up interfaces. See your vendor's ifconfig manual page for more details.

IP Version 6

Until now we have been discussing IPv4 addresses that are four octets long. The discussion in "Classless IP addressing" showed a clever way to extend the life of the 32 bit IPv4 address space. However, it was recognized long ago, even before the introduction of the World Wide Web, that the IPv4 address space was under pressure. IP Version 6 (IPv6) has been defined to solve the address space limitations by increasing the address length to 128 bit addresses. At the time of this writing, while most installed systems either do not support it or do not use it, most marketed systems support IPv6. Since it seems inevitable that you'll encounter some IPv6 networks in the next few years, we will explain some of the basics of IPv6. Note that IPv6 is sometimes referred to as IPng: IP Next Generation.Instead of dotted quads, IPv6 addresses are usually expressed as: 

x:x:x:x:x:x:x:x


where each x is a 16 bit hexadecimal value. In environments where a network is transitioning from IP Version 4 to Version 6, you might want to use a form like: 

x:x:x:x:x:x:d.d.d.d


where d.d.d.d represents an IP Version 4 dotted quad.When there are one or more consecutive sequences of x's such that each x is all zeroes, the sequence can be replaced with "::", but there can be only one such "::" abbreviation in an IPv6 address. Thus: 

:0000:5678:9ABC:DEF0:1234:5678:9ABC 3:0:0:0:0:0:3333:4444


can be abbreviated as: 

::5678:9ABC:DEF0:1234:5678:9ABC 3::3333:4444


As you might expect, IPv6 dispenses with address classes for unicast addresses. You specify classless network numbers (address prefixes), using the same classless addressing notation that IP Version 4 uses.

IP Version 6 address pools

While the designation of the network number in IPv6 is classless, the 128-bit address is still carved up into various pools. Portions of the address space are allocated for:

IP Version 6 loopback address

Instead of dedicating about 16 million addresses for loopback interfaces as IPv4 does, IPv6 uses just one address for that purpose: 

::1


IP Version 6 unspecified address

IPv6 introduces the concept of an "unspecified" address, which is all zeroes: 

::0


This address can be used by hosts that don't know their own address, but need to generate queries to determine their address assignment. Such hosts would use "::0" as the source address in an IPv6 packet.