The Internet



A few explanations are in order before connecting to the Internet. Time spent learning before hand will save twice the troubleshooting time later.
Internet Protocol (IP):
TCP/IP is the suite of networking protocols used to communicate on the internet. IP uses "Addresses" and "Subnet Masks" to define separate hosts (IP connected devices) and networks. Each host on a network must have a unique IP address. IP addresses are most commonly seen in dotted decimal notation i.e. 192.168.1.1. Each number separated by a "." is known as an octet, and can range in value from 0 to 255. Several address ranges have been set aside as private and cannot access or be accessed from the internet, addresses starting with 10, 172.16.to 172.31 , or 192.168 are private.
For performance (and other) reasons the internet can't be one big flat network, so subnet masks are used to separate IP addresses into separate networks. I'll just say that all hosts on a network must have the same subnet mask to communicate, and that 255.255.255.0 is a good mask for a home network. Once a network is determined by the IP address/subnet mask pair two addresses are not available to the network hosts. The lowest address is the "Network Address" and is used to definte the network, the highest address is the "Broadcast Address" and used for communication to all hosts on the network. Using the 255.255.255.0 subnet mask an address with .0 as the last octet would be the network address and .255 would be the broadcast address, leaving room for 254 hosts numbered from .1 to .254.
In most cases the ISP will automatically give the host an IP address and subnet mask when it connects and that address may be different every time it connects. This is known as "Dynamic Addressing".
This site has a pretty in depth but easy to understand tutorial on IP addressing.
One more thing that needs to be covered is the concept of ports. IP ports are not switch or router ports, they are a means of connecting the packet to an application. Without going into too much detail I'll try to illustrate the point with an example. When I want to connect to Google my browser starts the connection. The OS will assign an unused port (just a number from 1 to 65535) for the transaction, since Google is a web site port 80 will be included in the packet header as the destination port and the assigned port as the source port. Since the destination port is 80 Google's server knows that it is destined for it's web server. Google then sends its reply back to my computer using my port as the destination and 80 as the source. When my computer decodes the packet it sees that my browser is using that port so it sends it on to the browser, if no reply comes back the port will eventually be closed and become available for reuse. This is what allows the computer to check e-mail and browse the web at the same time. IANA assigns the commonly used port numbers.
Domain Name Service (DNS):
If every internet connected server has a unique IP address, how do we find the one we want? That's what DNS does. When your browser wants to connect to Google.com it asks a DNS server for the address. If your browser is stuck and the status bar says "finding www.google.com" then you have a DNS problem. DNS also allows google to be found if they put up a new server on a different address.
Modems and bridges:
These two items are lumped together because most cable modems and dsl modems are actually bridges. Bridges transparently connect two different network segments. The networks can be separated by the physical layer (i.e. ethernet to home PNA) or at the network layer. Bridges at the network layer have really evolved into routers. Most cable and dsl modems simply convert the source signal from the ISP's method into ethernet or USB signals for use by your computer or lan. Modems (Modulator/Demodulator) were developed to convert digital signals into analog signals used over telephone lines.
Routers:
Routers are used to connect different networks. The most common type of router used in homes is a simple nat router. Since most of these have a built in switch they make it easy to share a broadband connection between several computers. The router automatically assigns a private address to all of the computers behind it and holds the public (global) IP address for its WAN (Wide Area Network) port. Please see the NAT section of the BBR Security FAQ for a more in depth look at these devices, their strengths and limitations. Since they prevent worms from reaching your computers and now cost about the same as a trip to the movies for 2 people one of these devices is worthwhile whenever they can be used.
Switches:
Switches connect devices on the same network. They work by memorizing the MAC (Media Access Control) address of every connected device on the network. When computer A needs to talk to computer B the switch passes the communication between the appropriate switch ports. Switches are an evolution of the hub which repeated an incoming packet to all hub ports. With a switch computers A and B can be transferring files between each other while computer C is connected to the internet on router D with no interference.