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By Root 429 0

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 book, 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):

192.9.200.1

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 Section 1.3.3 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 book.

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