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Email to an IP address, Domain Name Service, DNS lookups,
Reverse lookup, Advanced DNS, Traceroute, SMTP Relay

What if the site is being coy
and trying to hide their domain name?

Most virtual web-hosting companies require customers to have a domain name, but if it's not used anywhere and the website only uses it's IP address to advertise, then it is not easy to be found.

Other times a pool of IP addresses is shared between a number of machines - eg on a dynamic-IP dialup connection your machine will be allocated a different IP address each time you connect.

These addresses are usually written in Dotted Quad notation, as a series of four 8 bit numbers, written in decimal and separated by periods. For example 151.196.75.10

Each number is in the range 0 to 255 - so if you ever see something that looks like an IP address with numbers outside those ranges it's not a real address.

The leftmost number is the most significant and the rightmost the least. So... 151.196.75.10 and 151.196.75.11 are right next-door to each other whilst 151.196.75.10 and 152.196.75.10 are completely unrelated.

IP Address Nslookup is a tool to find your server IP address. Use windows DOS "Command Prompt" (click Start/Programs/Accessories), when the DOS screen opens, at the prompt enter nslookup followed by the name of the domain.

Example - if your domain is mailsbroadcast.com enter; Nslookup mailsbroadcast.com [enter] to display your domain IP address.

To display a DOS prompt in Windows 2000 or XP,
go to Start/Run, type in "CMD" and click OK.

IP Address allocation

Traditionally IP addresses were allocated to Countries, companies or ISPs in blocks. Therefore, when an email is sent from a mail server machine to a recipient mail server machine it is quite easy to detect where or which country the mail is coming from because the IP address is attached into the message header.
 

A Class A Address Block or, less formally, an A Block is a block of 16,000,000 or so (2^24) addresses from X.0.0.0 to X.255.255.255, where 0 < X < 127.

So... the entire 9.0.0.0 - 9.255.255.255 range of addresses is the A Block owned by IBM.

A B Block is a block of 65,000 or so (2^16) addresses from X.Y.0.0 to X.Y.255.255, where 127 < X < 192 and 0 <= Y < 256

A C Block is a block of 256 addresses from X.Y.Z.0 to X.Y.Z.255 where 191 < X < 224, 0 <= Y,Z <256

(There are also D and E class addresses allocated in the 224-255 range - these are reserved for multicast and experimental applications - you'll never see them in practice)

Traditional blocks are often described using the first address in the block, eg IBM own A Block 9.0.0.0 and Cyberpromo own C Block 205.199.2.0 Other times they may be described using just the constant prefix, eg net 9 for IBM or net 205.199.2 for CyberPromo

[You'll often hear any address range from X.Y.Z.0 to X.Y.Z.255 called a C Block even though it technically isn't unless 191 < X < 224]

CIDR Allocation
Recently things have changed. IP addresses are in short supply and routers have become more sophisticated, so it's now usual to allocate blocks of addresses on pretty much any bit boundary.

You'll often see blocks of 64 addresses for instance, such as 151.196.75.128 to 151.196.75.191

A common way of naming these blocks is CIDR syntax - this is the initial constant prefix and the length in bits.

So... 151.196.75.0 to 151.196.75.255 might be described as 151.196.75/24 and... 151.196.75.128 to 151.196.75.191 might be described as 151.196.75.128/26

Special Cases
There are some special ranges of addresses reserved for special uses.

Loopback
The 127.*.*.* block is reserved for local loopback - so these addresses will always point back to your own machine. The canonical loopback address is 127.0.0.1

Private networks
Some blocks of addresses are allocated for private networks - packets from these machines should be dropped by most routers. Why is this useful? If you want to setup a private network you don't need to use up any of the scarce allocation of 'real' IP addresses. So you need to make up your own addressing scheme to use internally.

As long as you're not connected to the internet in any way you could use any scheme you wanted. But what if you have a gateway machine that lets you e.g. send mail to and from the internet?

If you chose an arbitrary IP address range for your local network and the packets leaked through the gateway onto the internet they'd end up going to the Real owner of those addresses, probably spam their system and provoking stern 'phone calls.

This has happened, even within huge multinational corporations who didn't follow the rules and chose arbitrary addresses for their internal network. So there are ranges of addresses allocated as private addresses.

You can use these quite safely, as everyone's routers are told to just throw packets to or from these addresses away. So if your packets escape they'll be deleted.

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