DCU's block of addresses

DCU topology

DCU subnets

DNS Lookup

Hosts and IP addresses

Machines have numbers that describe their place within the actual network topology: 

  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."

Host or machine name

 
	www.computing.dcu.ie 

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

	fileserver.salesdivision.regionaloffice.company.co.uk
Case irrelevant.

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.

Domain name space.

DCU's block of addresses

DCU has the following block of addresses: 
Dublin City University (NET-DCU-NET)
   Glasnevin
   Dublin, 9
   IE

   Netname: DCU-NET
   Netblock: 136.206.0.0 - 136.206.255.255
i.e. room for 2562 = 65,536 addresses.

DCU addresses run from:
136.206.0.0     to:
136.206.255.255
In binary, from:
1000 1000 1100 1110 0000 0000 0000 0000     to:
1000 1000 1100 1110 1111 1111 1111 1111
See 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.
See also here.

Subnets

Typically on a campus, each dept. might have its own LAN:

Above, each Ethernet is called a subnet.
Whole campus just appears as one network to outside world.
Campus main router has to route to correct Ethernet.

Subnet mask

Some bits are taken away from the host number and used as the subnet number. e.g. A 6 bit subnet number would allow for 64 Ethernets.
Main router is simple - It doesn't need to know about every host in system. It just needs to forward to the correct Ethernet.
Subnet mask is used to indicate the split in the host number bits:


Above: 6 bit subnet number. 10 bit host number on subnet.
Subnet mask: 1111 1111 1111 1111 1111 1100 0000 0000
= 255.255.252.0

Outside the organisation (the campus), the subnetting is not visible. So the organisation can change its subnet organisation without informing anyone.

Example

Organisation owns 130.50.*.*
6 bit subnet number (64 subnets). 10 bit host number on subnet (1024 hosts on each subnet).
(As in diagram above)

Address of Subnet 1 = 130.50.4.0 = 1000 0010 0011 0010 0000 0100 0000 0000 = (network address) (subnet 1) (0)
Subnet 1 starts at 130.50.4.1 = 1000 0010 0011 0010 0000 0100 0000 0001 = (network address) (subnet 1) (host 1)
runs up to 130.50.7.255 = 1000 0010 0011 0010 0000 0111 1111 1111 = (network address) (subnet 1) (host 1023)

Address of Subnet 2 = 130.50.8.0 = 1000 0010 0011 0010 0000 1000 0000 0000 = (network address) (subnet 2) (0)
Subnet 2 starts at 130.50.8.1 = 1000 0010 0011 0010 0000 1000 0000 0001 = (network address) (subnet 2) (host 1)
runs up to 130.50.11.255 = 1000 0010 0011 0010 0000 1011 1111 1111 = (network address) (subnet 2) (host 1023)

Address of Subnet 3 = 130.50.12.0 = 1000 0010 0011 0010 0000 1100 0000 0000 = (network address) (subnet 3) (0)
Subnet 3 starts at 130.50.12.1 = 1000 0010 0011 0010 0000 1100 0000 0001 = (network address) (subnet 3) (host 1)
runs up to 130.50.15.255 = 1000 0010 0011 0010 0000 1111 1111 1111 = (network address) (subnet 3) (host 1023)

Address of Subnet 4 = 130.50.16.0 = 1000 0010 0011 0010 0001 0000 0000 0000 = (network address) (subnet 4) (0)
Subnet 4 starts at 130.50.16.1 = 1000 0010 0011 0010 0001 0000 0000 0001 = (network address) (subnet 4) (host 1)
and so on ....

(Note on some older protocols subnet all 0's and subnet all 1's were reserved. On newer protocols they are not.)

A router on subnet k knows about hosts on local subnet k and about routers for other subnets (it does not know about hosts on other subnets).
It has a table of addresses: (network address) (subnet k) (host) telling how to get to a host on the local subnet
and: (network address) (other subnets) (0 only) telling how to get to that other subnet. No info about hosts on other subnets is held.

Example: Packet addressed to: 130.50.15.6 = 1000 0010 0011 0010 0000 1111 0000 0110 = (network address) (subnet 3) (host 774)
If this is subnet 3, the IP address will be in the routing table and the packet will be sent directly to the host.
Else we need to send to another router:
AND with subnet mask (above): 1111 1111 1111 1111 1111 1100 0000 0000
= 1000 0010 0011 0010 0000 1100 0000 0000 = (network address) (subnet 3) (host 0)
i.e. just change last 10 bits to 0
= 130.50.12.0
This is found in routing table as address for subnet 3. Packet is sent on to that subnet, for eventual forwarding to host.

In general:
(IP Address) AND (Subnet Mask) = (Subnet Address)

DCU topology

DCU and UCD

DCU subnets

First 16 bits are the DCU network number 136.206.
The DOS command:
$ ipconfig
shows:
Subnet Mask ... 255.255.255.0
i.e. 8 bit subnet number.
i.e. 256 possible subnets here, each with maximum 256 machines:
136 . 206 . (subnet) . (host)

Check your IP address. You'll find different subnets being used from room to room in CA.
i.e. Multiple Ethernets within CA alone.

Each node is a full Internet node (IP address). Doesn't really matter which Ethernet you are on.
Though may be useful to divide organisation into fixed groups so can easily restrict access to web page based on IP address, etc.

e.g. at time of writing:

Subnet Use Users
10 LG01, L101, L128, L201 undergrads
11 UNIX servers, www, mailhost staff, postgrads
17 L114, L129, L130 undergrads
18 LG25, LG26, LG27, LG28, L125 undergrads
19 postgrad machines postgrads
115 staff machines staff
218 wireless LAN staff, postgrads, undergrads

Apart from the users, there is also one file server machine on every subnet.

Different forms of URL

My address is:
http://www.computing.dcu.ie/~humphrys/
or:
http://computing.dcu.ie/~humphrys/
or the old:
http://www.compapp.dcu.ie/~humphrys/
These are currently aliases for the machine:
http://elbrus.computing.dcu.ie/~humphrys/
or the old:
http://elbrus.compapp.dcu.ie/~humphrys/
What all these translate to is:
http://136.206.11.240/~humphrys/

URL obscuring

Strangely enough, the above is also the same as:
http://2295204848/~humphrys/
(This only works if you don't go through the proxy.)

See:

Another fun thing you can try is something like this:

http://www.whitehouse.gov@2295204848/~humphrys/

http://www.whitehouse.gov@136.206.11.240/~humphrys/

This might even work (try UNIX browser, no proxy):

http://www.whitehouse.gov\security-staff\id-no\@2295204848/~humphrys/

http://www.whitehouse.gov\security-staff\id-no\@136.206.11.240/~humphrys/

Sub-spaces v. Actual hosts

Historically: 

	ibm.com
would refer to a sub-space of the Net, rather than to an actual machine. Actual machines would be called 
	*.ibm.com
i.e. things like: 
	www.ibm.com
	ftp.ibm.com
	researchlab.ibm.com
	sales.texas.mainframedivision.ibm.com
Now, because so many people type: 
	ibm.com
into web browsers, browsers are set up to redirect these to: 
	www.ibm.com
More than that, because so many people type: 
	http://ibm.com
companies are setting up that address to point to an actual machine as well as being the name of a sub-space.

http://computing.dcu.ie   does this now.

This alias is done at DNS level, not browser level. (Note that http://computing.dcu.ie works but http://compapp.dcu.ie doesn't.)