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  1. 1. Data Networking Year 2 Classless Networks Colm Bennett
  2. 2. Classless networks - Why? <ul><li>Preceded by Class networks and then subnetting </li></ul><ul><li>Only ~16,000 Class B's </li></ul><ul><li>Subnetting helped with management of these </li></ul><ul><li>But didn't help with organisation's taking them when they didn't need all ~65,000 addresses </li></ul><ul><li>In fact subnetting encouraged the use of Class B's as they were now easier to manage than mulitple Class C's </li></ul>
  3. 3. Classless networks - Why? <ul><li>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 </li></ul><ul><li>The answer – drop classes altogether </li></ul>
  4. 4. Classless Inter Domain Routing <ul><li>CIDR (“Cider”) </li></ul><ul><li>Forget about Classes! </li></ul><ul><li>Makes blocks of addresses assignable </li></ul><ul><ul><li>Complete flexibility over number of bits used for network part </li></ul></ul><ul><li>These blocks can be further split into smaller blocks </li></ul><ul><li>EFFECTIVELY DIFFERENCE IS THAT NETWORK BOUNDARY CAN BE AT ANY BIT </li></ul>
  5. 5. Remember - Alternative Notation <ul><li>“Slash” notation puts a number after the IP Address indicating how many bits will be used for the Network address </li></ul><ul><li>Sometimes called the “Prefix” as it indicates the IP address is prefixed with a certain number of Network bits </li></ul><ul><li>So /24 means the first 24 bits are the Network part </li></ul><ul><ul><li>Same as subnet mask of </li></ul></ul>
  6. 6. CIDR Advantages <ul><li>Allows for routes to be aggregated in routing tables </li></ul><ul><ul><li>If address space is contiguous, no need to refer to each Class C or B network individually </li></ul></ul><ul><ul><li>e.g. 4 Class C networks </li></ul></ul><ul><ul><ul><li> /24 </li></ul></ul></ul><ul><ul><ul><li> /24 </li></ul></ul></ul><ul><ul><ul><li> /24 </li></ul></ul></ul><ul><ul><ul><li> /24 </li></ul></ul></ul><ul><ul><li>Can be aggregated as 192.168.4 /22 </li></ul></ul>
  7. 7. CIDR Advantages <ul><li>Allows for more granularity in assigning blocks of addresses to organisations </li></ul><ul><li>E.G. A 5000 host company can be given a /19 block (~8000 hosts) instead of a Class B (/16 ~65000 hosts) </li></ul>
  8. 8. CIDR Blocks <ul><li>CIDR blocks are shown using the slash notation (i.e. With a network prefix number) </li></ul><ul><li>A specific IP address with a network prefix number is a designation for a block of IP addresses starting from that address </li></ul><ul><ul><li>The size of the available block depends on the CIDR number </li></ul></ul><ul><ul><li>Number is the network part so what remains is the host part </li></ul></ul><ul><ul><li>e.g. /18 leaves 14 bits for hosts, 2^14 -2 = 16382 Hosts </li></ul></ul><ul><ul><li>Or could be “subnetted” as normal into smaller subnets </li></ul></ul>
  9. 9. Valid CIDR Blocks <ul><li>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 </li></ul><ul><li>In other words, the host part of the address should be zero </li></ul><ul><li>This is obvious for Class A/B/C network blocks as the host has whole octets </li></ul><ul><ul><li>e.g. /24 or /8 </li></ul></ul><ul><li>Less obvious for other blocks when seen in isolation </li></ul><ul><ul><li>Is /20 valid? </li></ul></ul>
  10. 10. Valid CIDR Blocks <ul><li> /20 </li></ul><ul><ul><li>1000 1100.0000 0101.1001 0 1 00.0000 0000 </li></ul></ul><ul><ul><li>The host part is not zero </li></ul></ul><ul><li>Quick way to check is that the number should be divisible by the value of that octet with just the first network bit set </li></ul><ul><ul><li>So here the 3 rd octet has 4 network bits </li></ul></ul><ul><ul><ul><li>0001 0000 (16) would be the first valid value </li></ul></ul></ul><ul><ul><ul><li>So each subsequent boundary must be divisible by this </li></ul></ul></ul><ul><ul><ul><li>148 is not divisible by 16 </li></ul></ul></ul>
  11. 11. Valid CIDR Blocks <ul><li> /22 is however valid </li></ul><ul><li>/22 means we are using 6 bits from 3 rd octet </li></ul><ul><ul><li>So first valid boundary is 0000 0100 = 4 </li></ul></ul><ul><ul><li>148 is divisible by 4 </li></ul></ul><ul><li>This is more obvious in real life because you will assigning blocks from larger blocks </li></ul>
  12. 12. Real Life Use <ul><li>Large blocks of addresses are given to regional controlling organisations </li></ul><ul><ul><li>RIPE for Europe </li></ul></ul><ul><li>These in turn allocate blocks to major “Tier 1” ISPs/Organisations in each country in the region </li></ul><ul><li>Main ISPs will further allocate blocks onto other smaller ISPs or organisations </li></ul>
  13. 13. Example <ul><li>Check </li></ul><ul><ul><li>What block is it allocated out of? </li></ul></ul><ul><ul><li>Who controls that block? </li></ul></ul><ul><ul><li>What block was that block allocated out of? </li></ul></ul><ul><ul><li>Do we actually own the whole block? </li></ul></ul><ul><li>Use online DIG and RIPE database </li></ul>