This document is intentionally limited in scope; meaning I include or eliminate any details I feel necessary for clarity and brevity.
Here, we are concerned with computers tied together with coax cable or other electrical means, on which packets of information flow. We will also limit the discussion to TCP/IP type Ethernet packets, although the electrical cable where the packets travel does not prefer these over any other transmission type.
Ethernet describes how each packet of information is assembled for transmission, but does NOT describe the electrical aspect of transmission. Ethernet packets can be broadcast over coaxial cable of many types, fiber optic lines, satellites, phone lines...etc. We will focus on coaxial cable transmission.
Ethernet breaks files you might send into small pieces, called packets. The basic Ethernet packet consists of (substantially simplified):
On a global scale there could be billions of machines (called HOSTS), all attempting to communicate to each other. For our discussion we'll assume the norm of two machines in mutual contact. The problem is to assure that each machine has a unique address, and that there are enough spare addresses to cover future expansion.Accordingly, a 48 bit address scheme was created, allowing 281 trillion machines unique identification. A given manufacturer of machines or networks cards would be assigned a block of numbers - enough to cover a lifetime of production. When transmitting between machines, eventually the Ethernet address must be known or acquired. The address takes the form of:
each set of hh represents an octet or 8 bits of the total address. An address example would be: 3d:f4:01:77:3c, uniquely identifying a machine.
The Internet address allows easier control over smaller isolated networks and their machines, like your company or college computer network. It also allows one machine - a server for example - to take on many Internet addresses, which reference a single Ethernet address. There are many other obscure reasons why this second address is present, none of which apply for our discussion. Most of time you will only know or must acquire the Internet address . The Ethernet address is usually hidden from the user. The 32 bit Internet address takes the form of:
where each d is a decimal number from 0 to 255. The address does not translate directly to a given machine, but according to CLASS rules, where:
Class A describes a isolated network where there can be up to 16.7 million machines. The octet describes the network number, the last three octets the number of machines in the network. The address form:
network.machine.machine.machine
Class B describes a network where there can be up to approx. 65,000 machines. The first two octets indicate the network, the last two octets the machine. The address form:
network.network.machine.machine
Class C describes a network, where there can be up to 256 machines. The first three octets indicate the network, the last octet the machine. The address form:
network.network.network.machine
From: | To: | Class | Machines(per net) | Networks | ||||||||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
0.0.0.0 | 127.255.255.255 | Class A | Up to 16.7 million machines | 128 nets
128.0.0.0 | 191.255.255.255 | Class B | Up to 65,536 machines | 16,383 nets
| 192.0.0.0 | 223.255.255.255 | Class C | Up to 256 machines | 2.1 million nets
| 224.0.0.0 | 239.255.255.255 | Class D | Multicasting
| 240.0.0.0 | 247.255.255.255 | Class E | Reserved for future use
| |
Most networks, obviously, are Class B or Class C, with Class B nets being mostly used by large companies, universities, government, etc., and Class C nets by everybody else. Tying classes and the Internet address together; the Internet address 192.1.1.5 is a Class C address. It decodes as a network ID = 192.1.1, and a machine ID on that network of 5. In a simple system, only those machines with addresses beginning in 192.1.1 could communicate with the example machine.
Any two machines wishing to network over a simple system must have the SAME Internet network number, but must have UNIQUE Internet machine numbers.
The user data portion of each packet contains some other system information, then, of course, a portion of the file you are sending or receiving. Since your file has been broken into small chunks (usually 1024 byte chunks), a portion of the system information is for tracking which packet of which file is being sent/received. It is possible that packets can be lost or transmitted out of order, requiring the receiving host to assemble the data in the correct order.
The 605 sends Ethernet packets via 50 ohm coaxial cable. A coaxial system may have many "legs" or runs from a central distribution box, forming a star-like network. The general rules for coaxial: