Hosts and IP addresses
Machines have numbers that describe their place
within the actual network topology:
Four 8 bit
= 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:
Numeric - 4 parts.
Text - Can have variable number of parts.
"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."
It assigns chunks of the address space
to regional (e.g. international) authorities,
who then assign addresses in those regions.
Host or machine name
(actual machine).(organisation subdomains).(international subdomains)
Case of machine name
(Case of the machine name part of a URL
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:
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
Domain name space.
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.
country codes for vehicles.
- 1924: Ireland adopted "SE" for vehicles in 1924 ("Saorstat Eireann").
Iran adopted "IR" for vehicles in 1936.
In retrospect, that was the moment Ireland lost it for the Internet.
Ireland switched to "EIR" for vehicles in 1938 ("Eire").
Ireland switched to the English version in 1962.
"IR" was taken,
so Ireland switched to "IRL", which it still has today.
strictly 2 letter and
strictly 3 letter
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".
Internet country code top-level domains were defined in 1985, based on the ISO 3166
2 letter codes.
Hence we got the
registration of the
DCU has the following block of addresses:
Dublin City University (NET-DCU-NET)
Netblock: 18.104.22.168 - 22.214.171.124
i.e. room for 2562
= 65,536 addresses.
DCU addresses run from:
In binary, from:
1000 1000 1100 1110 0000 0000 0000 0000
1000 1000 1100 1110 1111 1111 1111 1111
IP decimal-binary table
First 16 bits are the DCU network number 136.206.
This is in binary:
1000 1000 1100 1110
Second 16 bits are the host number on that network.
This is a
Class B network.
To be precise, the leading 10 indicates Class B,
then the network number is the 14 bit:
00 1000 1100 1110
So a DCU address is:
Class B identifier, DCU network, machine number n:
10 00 1000 1100 1110 nnnn nnnn nnnn nnnn
IP address shows network class.
Special IP addresses.
Reserved IP addresses
Map of IPv4 address space
by Randall Munroe
Complications - CIDR, NAT, IPv6
- IPv4 address exhaustion
- Assigning blocks has become more complex.
- DCU got its block under the old system (before CIDR in 1993).
- 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
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.
has 128-bit addresses =
= 340 trillion trillion trillion.
- Recall current IPv4 has 32-bit addresses =
= 4 billion.
My address is:
These are currently aliases for the machine:
What all these translate to is:
Decimal IP address notation
Strangely enough, the above is also the same as:
This may or may not work:
- It may not work if you go through the DCU proxy.
- The Google Chrome browser
may translate it to the normal IP before even making the request.
Q. Why does this one lead to my web page?
Embed password in URL
For a page that needs a password, you can
embed the password in the URL
This is perfectly valid, but also gives us a new way of obscuring URLs.
Try these on different Linux and Windows browsers:
These could all lead to a numeric URL which fakes the look of a PayPal login page.
Q. How to be safe?
A. Never click on links in unsolicited email.
Domain name = Host name
This is a domain:
and these are hosts in the domain:
But as the Web developed, people wanted to be able to drop the "www" part,
so it is common to set up this:
as an alias for this:
This alias is done at DNS level.
If no DNS alias exists, the browser may or may not do it for you.