1. IP Addressing - The Problem
Have to assign addresses so that the Internet can find a destination with the minimum of processing, memory, bandwidth etc
Therefore address must be assigned so that we can quickly identify the rough location of a machine
ie, address must be based on the home network

2. IP Addressing - The Problem
IPv4 addresses begin with the address of the network where machine is located
Allows routers to figure out quickly where the machine is located
Once a packet has reached this network, it is the responsibility of the network to find the correct machine (and send the packet there)

3. IP Addressing - The Problem
We do not want to waste addresses
Therefore we do not want to allocate to any network, a lot of addresses which will never be used
However, we do want to leave room for growth of the networks
So must leave some unused addresses for every network

4. IP Addressing - The Problem
Networks are of different sizes
Smallest may be just a few computers
Largest may have hundreds of thousands
How do we differentiate between networks of different sizes?

5. IPv4
The solution adopted by IPv4 was to have several “classes” of networks
Class A networks - up to 224 = 16,000,000 addresses
Class B networks - up to 216 = 65,000 addresses
Class C networks - up to 28 = 256 addresses

6. IPv4 IPv4 Address Formats 0 Network (7 bits) Host (24 bits) Class A
10 Network (14 bits) Host (16 bits) Class B
110 Network (21 bits) Host (8 bits) Class C
IPv4 Address Formats

7. IPv4 This gives very coarse granularity However, does allow:
many small networks = 221 = 2,000,000
moderate number of medium sized networks = 214 = 16,000
very few large networks = 27 = 128 (less than one per member country of the UN)

8. IPv4 When the Internet was small, the coarseness was not a problem
Now we are running out of addresses
This system locks up addresses that are needed in other parts of the network
We need to get out of this somehow

9. Subnetting
The Internet community has solved this problem in three steps
1 Class-Based IPv4 Subnetting
2 Classless Inter-Domain Routing (CIDR)
3 Distributed subnetting - IPv6

10. Class-Based IPv4 Subnetting
Remember the structure of the address: class identifier.network id.host id
Problem is that the boundary between fields for network and host ids can only move in steps of eight bits
Would like to let it move in smaller steps

11. Class-Based IPv4 Subnetting
We cannot move the boundary back towards the beginning of the address
We can move it forwards, using class-based IPv4 subnetting
We use the first few bits of the host id as the identifier of a new network which we call a “subnetwork”

12. Class-Based IPv4 Subnetting
We need a number of networks to agree to share a network ID, and to use different subnetwork IDs
eg, a Class B network has 65,000 addresses. If 12 networks had an average of, say, 2000 hosts on their networks, but were all too big to use a Class C network ID, they would apply for a class B network ID

13. Class-Based IPv4 Subnetting
Any of them would waste a lot of address space if they were given a Class B network ID
But, together, they could share one network ID
Since there are 12 of them, we need four bits as the subnet ID (24 = 16 > 12)

14. Class-Based IPv4 Subnetting
10 Network ID Subnet ID Host ID
Address would now look like this
Class ID as before
Network ID as before
Subnet ID four bits
Host ID 12 bits

15. Class-Based IPv4 Subnetting
No of hosts allowed for one subnet is 212 = 4,096
The larger networks could be given more than one subnet ID
Would allow address space to be allocated in blocks of 4,096 addresses

16. Reserved Addresses
ID fields of all 0s or all 1s are not allocated to hosts
Subnet IDs cannot be all 1s

17. Class-Based IPv4 Subnetting
Host 1.1.2
Host 1.1.1
Subnet 1.1
Host 1.1.3
Network 1
Subnet 1.2
Host 1.2.1
Host 1.2.3
Host 1.2.2

18. Routing with Subnetting
Internet routers only look at the network ID
A single gateway (router) could be used for all these subnets
The gateway would then look at the subnet ID and send packets to the correct subnet
This is a good solution if all networks are within a small geographical area, eg a single building or city block

19. Routing to a WAN
Network could be a WAN, with all subnets owned by the same organisation
Each subnet would cover one location
Nearby routers could be informed of this situation
These routers could look at subnet ID and send packets to appropriate location

20. Classless Inter-Domain Routing
Variable length subnetting - within a single network ID, allow subnets with different length IDs (subnet masks)
Allows accommodation of different size subnets within the one network

21. CIDR
Every network which is given a block of addresses in CIDR must be listed in the routing table of all backbone routers
This can result in very large routing tables for these routers
There is no guarantee that these networks will be geographically close together

22. Network Address Translation
NAT is a quick and nasty solution to the problem of the shortage of IPv4 addresses
A single IP address is assigned to a network
Even if there are 10,000 computers on the network, they are all given the one IP address, as used by the network
This allows one address to cover 10,000 computers

23. N.A.T.
The problem arises when a packet arrives at the network from outside, ie from the Internet
How does the network’s router/gateway know where to send the packet?
(Usually each computer on the network has its own unique IP address.)
We need a NAT box at the router

24. N.A.T. Box 10.0.0.1 Address before translation
Address after translation
NAT box
To ISP’s router
Company router
Company LAN
Source: A.S. Tanenbaum

25. N.A.T. Packets leaving the network all have the same source address
Packets arriving at the network all have the same destination address, but must be sent to one of 10,000 different machines
We get around this problem by misusing the TCP or the UDP field

26. N.A.T.
It was observed that nearly all traffic between Internet networks uses either TCP or UDP as the transport layer protocol
This is the layer above the network layer (where the IP address is located) in the packet header
It is only used at the two ends of the connection, never in the networks which carry the packet

27. N.A.T.
Therefore it is (usually) safe for the NAT box to change the transport header, as long as it remembers to change it back
When an application establishes a connection with another machine, it nominates a “port” on its own machine and another port on the destination machine.

28. TCP Ports
The destination port tells the remote computer where to store an incoming packet
The remote computer does not use the source port for anything. It simply returns packets with this port number as the destination port
This allows us to use this port number to carry extra informaton

29. N.A.T. use of TCP ports
A packet from a computer in the home network carries its own IP address for use only in the LAN
The NAT records this address, and the TCP source port in a table
The line of the table is entered in the 16 bits of the TCP source port

30. N.A.T. use of TCP ports
The network IP address is written into the IP header in place of the source address
The packet is sent to its destination across the Internet, and returns to the router/gateway of the network
The router/gateway reads the 16 bits in the TCP header to find which line of its table to read

31. N.A.T. use of TCP ports
From the table, it finds the internal IP address of the machine for which the packet is intended, and also the correct TCP port to send the packet to
It then sends the packet to the correct machine
The machine knows which process to send the packet to (from the TCP header), and the connection is complete

32. Is NAT a Good Idea?
NAT uses TCP or UDP for a task it is not intended for
This produces many difficulties in practice
However, NAT provides us with a little extra time to get IPv6 into widespread use throughout the Internet

33. Supernetting
Organisations with complex networks can acquire contiguous blocks of Class C IDs (eg x00, x01, x10 and x11 where x = first 19 bits of Class C addresses) and advertise a single route for reaching all of them
Routers and gateways “advertise” their location to neighboring Internet nodes. This is used in routing

34. CIDR Network Naming
Internet Network Information Center (InterNIC) serves as the Internet central naming registry
With CIDR InterNIC delegated naming of local networks to ISPs and other middlemen

35. Use of Address to Locate a Destination
Router looks at first few bits of address to determine the class
Then looks at appropriate number of bits to determine the network ID
If network is known to router, sends packet on to appropriate next hop
Otherwise sends packet to “default router”

36. Default Router
Generally will be available router which is closest to the backbone
Routers in backbone do not have a “default router”
Must look at network ID and choose intelligent next hop
Must therefore have very large routing table

37. Backbone Router
This has become a big problem since there are 2,000,000 Class C IDs
CIDR has allowed Class C network IDs to be aggregated
So has taken some pressure off backbone routing tables
IPv6 has made it easier still

38. IPv6 Main problems with IPv4 are: Limited size of address space
Difficulty using network class system
Inflexibility in two level address (network.host)
InterNIC did all network naming
Size of routing tables in backbone routers

39. IPv6 Address Uses 128 bits (compare 32 bits for IPv4)
Represented as eight numbers divided by :
128 = 8*16, each number represents 16 bits
Numbers use hexadecimal system
eg 46F3:57:0:0:0:0:5D2C:21AA = 46F3:57::5D2C:21AA
(compare eg for IPv4)

40. IPv6 Address Types Unicast - specific physical interface to a network
Multicast - packets sent to all members of a set of physical interfaces
Anycast - packets sent to at least one member of a set of interfaces

41. Allocation of Addresses
Nearly all addresses are unassigned
Prefix 001 is used for “Aggregatable Global Unicast Addresses”
Accounts for 1/8 total address space
Prefix is used for multicast addresses
For other allocations, see RFC 2373

42. Aggregatable Global Unicast
These addresses (only) are formatted as follows
bits
FP TLA RES NLA SLA Interface ID
ID ID ID

43. Aggregatable Global Unicast
FP - Format Prefix - currently 001
TLA ID - Top Level Aggregation Identifier - contains the highest level routing information of the address. Currently 13 bits - limits routing table entries to 8,192
Res - eight bits reserved for future use

44. Aggregatable Global Unicast
NLA ID - Next Level Aggregation Identifier - to be used by organisations that control the top level IDs, eg large ISPs. Within their address space, they are free to configure up to 224 address sub-spaces
SLA ID - Site Level Aggregation Identifier - Each organisation can create its own internal hierarchical structure

45. Aggregatable Global Unicast
Interface ID - 64 bit field - Designed to use IEEE EUI-64 interface ID
Similar to 48 bit MAC address
Unique across global scope
264 interfaces = roughly 18 billion billion different addresses

46. Aggregatable Global Unicast
IPv6 addresses are allocated by the ISPs, and are based on the ISP structural hierarchy
IPv6 addressing is designed to help routers, and not to use all the theoretical 2128 possible addresses

47. ISP Hierarchical Structure
Internet backbone
Top Level ISP
Next Next Next Next Next Next Level Level Level Level Level Level ISP ISP ISP ISP ISP ISP

48. Routing with IPv6 Addresses
As before, routers have a default router
Send packets to the default router if they do not have a route to the TLA ID
Backbone routers do not have a default router
Must have a route to every TLA ID
There are only 8,192 TLA IDs

49. Routing with IPv6 Addresses
After packet has reached Top Level ISP, router looks at NLA ID.
All these NLA IDs correspond to next level ISPs which are clients of the top level ISP
This will be a relatively small number (although 24 bits are allowed at present)
Lower levels are treated similarly

50. Multicast Addresses
In both IPv4 and IPv6, multicast addresses are mapped to a set of unicast addresses
In IPv4, Class D is the class which contains all multicast addresses. The first four bits are 1110
In IPv6, the first eight bits are all 1s

51. Anycast Addresses
Packet is forwarded to at least one of the nodes which are members of the anycast address
Useful when any of the nodes will do the job
An example is a DNS (domain name server). It does not matter where the response comes from