ip addressing - the problem

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


• 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)


• 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


• 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?

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

IPv4

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

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)

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

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

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


• 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”


• 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


• 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)


• Address would now look like this

• Class ID as before

• Network ID as before

• Subnet ID four bits

• Host ID 12 bits

10 Network ID Subnet ID Host ID


• 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

Reserved Addresses

• ID fields of all 0s or all 1s are not allocated to hosts

• Subnet IDs cannot be all 1s


Host 1.1.1Host 1.1.2

Host 1.1.3 Subnet 1.1

Host 1.2.1

Host 1.2.2

Host 1.2.3

Network 1

Subnet 1.2

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

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

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

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

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

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

N.A.T. Box

Company LAN

Company router

NAT box

To ISP’s router

Address before translation

Address after translation

198.60.42.12

10.0.0.1

Source: A.S. Tanenbaum

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

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

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.

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

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


• 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


• 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

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

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

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

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”

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

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

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

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 223.182.21.93 for IPv4)

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

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 1111 1111 is used for multicast addresses

• For other allocations, see RFC 2373

Aggregatable Global Unicast

• These addresses (only) are formatted as follows

3 13 8 24 16 64 bits

FP TLA RES NLA SLA Interface ID

ID ID ID


• 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


• 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


• 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


• 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

ISP Hierarchical StructureInternet backbone

Top Level ISP

Next Next Next Next Next Next Level Level Level Level Level Level ISP ISP ISP ISP ISP ISP

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

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

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

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

ip addressing - the problem

Documents

class c network id

addressesipv40 network

new network

network idsince

class b network idbut

bitsclass a10 network

bitsclass b110 network

bitshost id