What are TCP/IP and IP addresses?
TCP/IP stands for Transfer Control Protocol / Internet Protocol, and is the commonest mechanism by which data is transferred over networks, including the Internet. When you connect a single computer to the Internet, your ISP allocates you an IP address - a single, unique address which identifies your address on the Internet for the duration of your call/connection. All TCP/IP packets sent to/from your computer contain this IP address. An IP address consists of four 8 bit numbers (represented in decimal format by a number between 0 and 155) separated by full stops. In addition to the IP address, every TCP/IP configuration must have the following:
A Subnet Mask, which tells the network how to distinguish between IP addresses that are part of the same network and those that belong to other networks.
A default gateway, which is a computer (or other device) that routes packets intended for addresses outside the local network
One or more Domain Name Servers, which are computers that translate domain names (such as billnot.com) into IP addresses
Public and Private IP Addresses
Any computer that is directly connected to the Internet needs a public IP address; one that can be reached by other computers on the Internet, so that information you request (Web pages and e-mail, for instance) can be routed back to your computer properly. When you connect to an Internet service provider, you're assigned a public IP address from a block of addresses registered to that ISP. If you use a dial-up connection, your ISP probably assigns a different IP address to your computer (drawn from its pool of available addresses) each time you connect. If you have a persistent connection to your ISP via ADSL or cable modem, your IP address will be permanent.
On a home or small office network, it's not necessary to have a public IP address for each computer on the network. In fact, configuring a network with all public addresses increases security risks and usually requires an extra fee from your ISP. A safer, less costly solution is to assign a single public IP address to a single computer (or a router or residential gateway). All other computers on the network connect to the Internet through that single address. Each of the computers on the local network has a private IP address that is not reachable from the outside world. To communicate with the Internet, the computer or router on the edge of the network uses a technology called network address translation (NAT) to pass packets back and forth between the single public IP address and the many private IP addresses on the network.
The Internet Assigned Numbers Authority (IANA) has reserved the following three blocks of the IP address space for use on private networks that are not directly connected to the Internet:
• 10.0.0.0-10.255.255.255
• 172.16.0.0-172.31.255.255
• 192.168.0.0-192.168.255.255
Routers, switches, and residential gateways that use NAT almost always assign addresses from these private ranges.
Every computer with Internet connectivity must have a unique IP address, so if you have many computers on your LAN, each must have its own local IP address. NAT (Network Address Translation) is a system for sharing a single Internet IP address amongst many local PCs. The NAT system automatically translate the PC's local IP address into the IP address being used for Internet access. Routers and Software-Routers typically provide an NAT facility. Despite being a complex operation, NAT generally works well, though not every application will work through a NAT router, so if you have particular requirements, so check with your supplier. Perhaps the main benefit of ' NAT' connections is that an external hacker can not connect directly to PCs on the private network and cause problems. This is because the local computers cannot be accessed directly by computers elsewhere on Internet, which can only 'see' the IP address of the ADSL router. But note that NAT is not resilient to all types of attack, so extra security is still recommended.
How does this all work? Tell me about YOUR set up
OK. My Internet (public) IP is 80.35.22.107 with a netmask of 255.255.255.0. That is set by Telefónica, and is fixed. Their DNS servers are 195.235.113.3 and 195.235.96.90. That is publicly available information. They also supplied me with an "IP de Gestion", a default local subnet setting for accessing their gateway from my router. Mine is 10.5.63.142 with a netmask of 255.255.254.0. This is theoretically alterable from my end, but I not only have no reason to do so, but I would be a fool to try. If I say that I don't fully understand the purpose of the IP de Gestion, I would only be being honest. My own local domestic network is in the 172.16.0.0 to 172.31.255.255 set, although I have limited this by setting my machines between 172.16.0.1 and 172.16.0.13 with a netmask of 255.255.255.240, making the chances of success for a malicious attack even less likely. My router is set up to be 172.16.0.1, and thus this becomes the setting for the default gateway for the machines on my net, which know nothing about the 80.35.22.107 address, simply sending and collecting everything from 172.16.0.1, the router, and leaving it to get on with its job.
Ports
One other technical term you're almost certain to come across is that of ports. A port is a sort of doorway through which specific type of information are exchanged. The TCP/IP protocol allows for 65535 ports, of which the first 1024 are called the "Well known ports". Of those 1024, you're only likely to come across a few, such as Port 25 for SMTP, Port 119 for NNTP, Port 80 for HTTP and 110 for POP3. The reason to have a basic understanding of what ports are is to understand how hackers attempt to break into your system. There are, as you can see, a great many unused ports. A hacker will "knock at the door" of one of these ports. If they are admitted, they can then explore your system. A firewall blocks all ports and all attempts at access except the once you say are OK (such as Internet Explorer using port 80). For a blocked port, the firewall will lock the door and tell the attacker to go away.
Words of wisdom from Bill Gray, an early adopter
If you want to know more.
Sangoma.com An advanced guide to TCP/IP
From Faqs.org , the RFC that defines TCP/IP
Port numbers - a comprehensive list for masochists