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You may find it helpful to think of the protocol layers as passing packets upstream and downstream in envelopes, where the packet envelope contains some protocol-specific header information but hides the remainder of the packet contents. As data messages are passed from the top most protocol layer down to the physical layer, the messages are put into envelopes of increasing size. Each layer takes the entire message and envelope from the layer above and adds its own information, creating a new message that is slightly larger than the original. When a packet is received, the data link layer strips off its envelope and passes the result up to the network layer, which similarly removes its header information from the packet and passes it up the stack again.

Ethernet addresses

Associated with the data link layer is a method for addressing hosts on the network. Every machine on an Ethernet has a unique, 48-bit address called its Ethernet or Media Access Control (MAC) address. Vendors making network-ready equipment ensure that every machine in the world has a unique MAC address. 24-bit prefixes for MAC addresses are assigned to hardware vendors, and each vendor is responsible for the uniqueness of the lower 24 bits. MAC addresses are usually represented as colon-separated pairs of hex digits:

8:0:20:ae:6:1f

Note that MAC addresses identify a host, and a host with multiple network interfaces may use the same MAC address on each.

Part of the data link layer's protocol-specific header are the packet's source and destination MAC addresses. Each protocol layer supports the notion of a broadcast, which is a packet or set of packets that must be sent to all hosts on the network. The broadcast MAC address is:

ff:ff:ff:ff:ff:ff

All network interfaces recognize this wildcard MAC address as a broadcast address, and pass the packet up to a higher-level protocol handler.

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 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