The OSI Networking Model is used as a reference point to describe
how the various "layers" of networking interoperate.
For this discussion, I will describe the bottom three layers:
Layer
Name
Protocols / Terms
Devices that operate in this layer
Addresses are called...
3
Network
IP, IPX, AppleTalk
Routers
Network Addresses
2
Datalink
Ethernet, Token Ring, PPP, SLIP, HDLC
Bridges, Switches, Repeaters, Hubs
Datalink, or MAC* addresses
1
Physical
Unshielded Twisted Pair, Shielded Twisted Pair, Coax, Twinax, Serial cable
Modems, CSU/DSUs
N/A (cables don't have addresses)
*MAC, in this case, stands for Media Access Control,
not to be confused with an address for a Macintosh...
Combinations that include a term from each layer describe fully
how a packet is getting from a given point "A" to a
directly connected point "B". For example, A may be
talking to B using IP over Ethernet over Unshielded Twisted Pair;
or, "my computer talks to my ISP using IP over PPP over a
serial cable" (a modem is simply a serial cable extender
in this sense.) From the physical layer standpoint, devices have
no addresses. On the datalink layer, all Ethernet and Token Ring
cards all have 6-byte addresses manufactured into them, called
MAC addresses (nothing to do with Macintoshes.) Point-to-point
links such as serial lines do not have MAC addresses, which creates
special cases from a data transmission standpoint, that are outside
the scope of this document.
The Physical layer defines the electrical media and signaling
used to transmit information on a wire (or wires.) The datalink
layer defines the format of the data as it is transmitted (e.g.,
an Ethernet frame.) Network layer information is encapsulated
inside datalink layer frames. If you look at an IP packet on an
Ethernet wire it would look something like this:
Ethernet Header (with dest and src MAC addr)
IP Header (with dest and src IP addr, and checksum)
Actual Data
Note that this indicates that, in order for two Ethernet-attached
stations to communicate with each other via IP, they must know
the MAC address of each other. If station "A" knows
the IP address of station "B", and knows station "B"
is on the same subnet, station "A" will issue an Address
Resolution Protocol (ARP) broadcast.
An ARP broadcast is a message that says, "Who out there is 192.168.1.1?"
The TCP/IP software running on the workstation or router at 192.168.1.1
is responsible for sending back an ARP response that says,
"I am 192.168.1.1, and my MAC address is 08:00:09:AF:24:33."
All stations keep an ARP
cache with the MAC and IP addresses of all the stations it recently
communicated with directly.
Try the command "arp -a" sometime
on a UNIX or Windows workstation; on a Cisco router, the command is "show arp".
Note that layer 1 devices are "invisible" to layer 2;
and layer 2 devices are "invisible" to layer 3. In other
words, TCP/IP doesn't care if you're running over Ethernet or
Token Ring, as long as it's connected properly. In fact, you can
put bridging and/or switching devices on your network without
disturbing any of your IP subnetting. Similarly, you can convert
between different types of media (e.g., coax to twisted pair) without
any layer 2 devices being aware of the change.
To change layer 1 media, you typically need
a layer 2 device (e.g., "I have a Ethernet Coax to Ethernet
Twisted-Pair repeater".) To change the layer 2 protocol (e.g.,
Ethernet to Token Ring) you typically need a layer 3 device (a
router.) All this is good, since it allows some measure of media
independence within the network; you can run IP over just about
anything better than two cans and a string, and even that, if
you can find transceivers to handle it ;-)
