Hosts and IP addresses

Machines have numbers that may (with some homework) describe their place within the network topology.
These are called IP addresses.
IP addresses under the standard IPv4 look like:

  106.132.204.106

Four 8 bit numbers.
256⁴ = 2³² = 4 billion.

Humans could never work with these. Since earliest days of networks, machines have text names, which describe their place within the logical hierarchy:

  www.mit.edu
  compsci.mit.edu
  techpapers.compsci.mit.edu

Numeric - 4 parts.
Text - Can have variable number of parts. Text syntax: "In theory, this subdivision can go down to 127 levels deep, and each label can contain up to 63 characters, as long as the whole domain name does not exceed a total length of 254 characters. But in practice some domain registries have shorter limits than that."

ICANN manages addressing.
It assigns chunks of the address space to regional (e.g. international) authorities, who then assign addresses in those regions.

Host or machine name

 
	www.computing.dcu.ie 

	(actual machine).(organisation subdomains).(international subdomains)

	fileserver.salesdivision.regionaloffice.company.co.uk

Case of machine name is irrelevant.
(Case of the machine name part of a URL is irrelevant.)
e.g. Using the ping tool at Unix command-line:

$ ping www.biscuits.com
www.biscuits.com is alive

$ ping WWW.BISCUITS.COM
WWW.BISCUITS.COM is alive

$ ping WWW.Biscuits.cOM
WWW.Biscuits.cOM is alive

$ ping www.biscuitss.com
ping: unknown host www.biscuitss.com

$ PING www.biscuits.com
PING: Command not found

The organisation can divide up its subdomains any way it likes. The organisation gets allocated a certain number of addresses, i.e. a subspace of the address space, such as:

   126.121.*.*

and can assign these any names it likes. It doesn't have to tell outside world (until an actual request is made).
See DNS Lookup.

Domains

Domain name space.

List of Internet top-level domains
Top-level domain
- Generic top-level domain
- country codes
- IANA official list of top-level domains
- Internationalized domain names (host names in non-Latin alphabets)
Proposed top-level domains
ICANN process to generate new top-level domains on an ongoing basis.
- e.g. A company gets its name as a TLD.
- Fees. Cost is $185 k.
- Applied-for strings

Why did Ireland get IE?

I always wondered where "IE" for Ireland came from. Why not "IR"?
Thanks to Feargal Fitzpatrick for helping work out the following story.
The story goes back to the 1920s.

See country codes for vehicles.
1924: Ireland adopted "SE" for vehicles in 1924 ("Saorstat Eireann").
1936: Iran adopted "IR" for vehicles in 1936. In retrospect, that was the moment Ireland lost it for the Internet.
1938: Ireland switched to "EIR" for vehicles in 1938 ("Eire").
1962: Ireland switched to the English version in 1962. "IR" was taken, so Ireland switched to "IRL", which it still has today.
1974: The ISO 3166 standard defined strictly 2 letter and strictly 3 letter country codes, based on the country codes for vehicles. Ireland therefore got "IRL" for its 3 letter code. For the 2 letter code, "IR" was taken, so Ireland became "IE".
1985: Internet country code top-level domains were defined in 1985, based on the ISO 3166 2 letter codes. Hence we got the .ie domain.
See 1988 post about the registration of the .ie domain.

Network classes (the old way of handing out IP addresses)

Map of IPv4 address space

From xkcd by Randall Munroe.

Modern IP assignment - CIDR, NAT, IPv6

IPv4 address exhaustion
CIDR
- Assigning blocks became more complex with CIDR in 1993.
- The network (or block) assigned can have any number of bits. "CIDR notation" shows how many bits it has.
- DCU got its block under the old system (with small number of network classes).
NAT
- On-the-fly translation from "internal" address spaces inside an ISP/organisation to externally-viewable IP addresses.
- Proxies - Check what other sites see as your IP address and it may show you using one of the DCU proxies rather than your actual IP address. Yet the data goes back to you only, and not to all other users of the proxy.
- DCU uses real IP addresses internally, but in fact your actual IP address need not be externally visible, and hence could be anything. With NAT, networks that previously required a reserved block of addresses can be connected to the Internet with as little as a single IP address.
IPv6
- IPv6 has 128-bit addresses = 2¹²⁸ = 340 trillion trillion trillion.
- Recall current IPv4 has 32-bit addresses = 2³² = 4 billion.

Different forms of URL: www.computing.dcu.ie

My address is:

https://www.computing.dcu.ie/~humphrys/

(This is now a redirect to my site.)
This is an alias for the machine:

https://corno.computing.dcu.ie/~humphrys/

IP address

The host above translates to 136.206.217.26.
The following may not work.
It is complicated by tilde re-direction and an SSL cert.

https://136.206.217.26/~humphrys/
http://136.206.217.26/~humphrys/

https://136.206.217.26/
http://136.206.217.26/

Different forms of URL: student.computing.dcu.ie

My address is:

https://student.computing.dcu.ie/~mhumphrysdculab/

This is an alias for the machine:

https://razor.computing.dcu.ie/~mhumphrysdculab/

IP address

The host above translates to 136.206.218.15.
Again, the following may not work:

https://136.206.218.15/~mhumphrysdculab/
http://136.206.218.15/~mhumphrysdculab/
https://136.206.218.15/
http://136.206.218.15/