Data Networking Year 2

Classless Networks

Colm Bennett

Classless networks - Why?

Preceded by Class networks and then subnetting

Only ~16,000 Class B's

Subnetting helped with management of these

But didn't help with organisation's taking them when they didn't need all ~65,000 addresses

In fact subnetting encouraged the use of Class B's as they were now easier to manage than mulitple Class C's

Classless networks - Why?

So the Class system left the internet caught between having relatively few Class B's assigned but then running out of Ips or assigning mulitple Class C's and severely clogging up routing lists and complicating local management

The answer drop classes altogether

Classless Inter Domain Routing

CIDR (Cider)

Forget about Classes!

Makes blocks of addresses assignable

Complete flexibility over number of bits used for network part

These blocks can be further split into smaller blocks

EFFECTIVELY DIFFERENCE IS THAT NETWORK BOUNDARY CAN BE AT ANY BIT

Remember - Alternative Notation

Slash notation puts a number after the IP Address indicating how many bits will be used for the Network address

Sometimes called the Prefix as it indicates the IP address is prefixed with a certain number of Network bits

So 192.168.5.0 /24 means the first 24 bits are the Network part

Same as subnet mask of 255.255.255.0

CIDR Advantages

Allows for routes to be aggregated in routing tables

If address space is contiguous, no need to refer to each Class C or B network individually

e.g. 4 Class C networks

192.168.4.0 /24

192.168.5.0 /24

192.168.6.0 /24

192.168.7.0 /24

Can be aggregated as 192.168.4 /22

CIDR Advantages

Allows for more granularity in assigning blocks of addresses to organisations

E.G. A 5000 host company can be given a /19 block (~8000 hosts) instead of a Class B (/16 ~65000 hosts)

CIDR Blocks

CIDR blocks are shown using the slash notation (i.e. With a network prefix number)

A specific IP address with a network prefix number is a designation for a block of IP addresses starting from that address

The size of the available block depends on the CIDR number

Number is the network part so what remains is the host part

e.g. 83.147.128.0 /18 leaves 14 bits for hosts, 2^14 -2 = 16382 Hosts

Or could be subnetted as normal into smaller subnets

Valid CIDR Blocks

A CIDR block is only valid if the IP address falls on a valid boundary for the prefix bits otherwise it is just a specific IP within the block

In other words, the host part of the address should be zero

This is obvious for Class A/B/C network blocks as the host has whole octets

e.g. 192.168.3.0 /24 or 10.0.0.0 /8

Less obvious for other blocks when seen in isolation

Is 140.5.148.0 /20 valid?

Valid CIDR Blocks

140.5.148.0 /20

1000 1100.0000 0101.1001 0100.0000 0000

The host part is not zero

Quick way to check is that the number should be divisible by the value of that octet with just the first network bit set

So here the 3rd octet has 4 network bits

0001 0000 (16) would be the first valid value

So each subsequent boundary must be divisible by this

148 is not divisible by 16

Valid CIDR Blocks

140.5.148.0 /22 is however valid

/22 means we are using 6 bits from 3rd octet

So first valid boundary is 0000 0100 = 4

148 is divisible by 4

This is more obvious in real life because you will assigning blocks from larger blocks

Real Life Use

Large blocks of addresses are given to regional controlling organisations

RIPE for Europe

These in turn allocate blocks to major Tier 1 ISPs/Organisations in each country in the region

Main ISPs will further allocate blocks onto other smaller ISPs or organisations

Example

Check killestercollege.ie

What block is it allocated out of?

Who controls that block?

What block was that block allocated out of?

Do we actually own the whole block?

Use online DIG and RIPE database

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