Global Internet Computer Networks Dr Jorge A Cobb Where Are We l l Internet Connect heterogeneous collection of networks Simple addressing hierarchy Scalability Challenges Several challenges exist to make the Internet of global scale We address several of these in these notes 2 Global Internet Summary of Topics l IP address hierarchy evolution Subnetting CIDR l Evolution of Internet structure l Virtual geographies l Networks Domains Autonomous Systems Routing with domains Intradomain routing Interdomain routing 3 Problems of Scale 1 Inefficient address allocation Most physical networks i e LANs have only about 100 hosts or less Assigning class A or B to them would be wasteful Solution subnetting 2 Too many networks for routing Networks at the core need to be aware of each class A B and C network number Too many networks Routing tables don t scale Solution CIDR l We will tackle each of these in turn i e we start with 1 above 4 Subnetting l l l Assume an organization is given a class B network Class B has two bytes for network and two bytes for the host E g 128 174 0 0 recall network s have 0 s for the host bits Think of this as an address block of 2 16 addresses First address in the block is 128 174 0 0 1 0 0 0 0 0 0 0 1 0 1 0 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 l Last address in the block is 128 174 255 255 1 0 0 0 0 0 0 0 1 0 1 0 1 1 1 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 l l Actually 2 16 2 addresses 128 174 0 0 and 128 174 255 255 cannot be used J I sometimes draw this block as follows 1 0 0 0 0 0 0 0 1 0 1 0 1 1 1 0 X X X X X X X X X X X X X X X X 5 Subnetting continued l Assume the organization has multiple physical networks with a few hundred hosts in each physical network l The class B network can be broken into smaller sub networks l Idea take a single IP network number Break its block of addresses into smaller blocks subnets Allocate a smaller block of IP addresses to each physical network Not all blocks are of the same size 6 Example l Consider again network 128 174 0 0 1 0 0 0 0 0 0 0 1 0 1 0 1 1 1 0 X X X X X X X X X X X X X X X X l One sub block of addresses could be as follows 1 0 0 0 0 0 0 0 1 0 1 0 1 1 1 0 1 0 0 0 1 1 1 0 1 X X X X X X X l l l l First 25 bits identify the network and the subnetwork The last 7 bits identify the host within the subnetwork This subnetwork is identified as 128 174 142 128 zero for the host bits It also has associated with it a subnet mask 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 1 network or subnet bit 0 host bit 7 Another subnet l Consider again network 128 174 0 0 1 0 0 0 0 0 0 0 1 0 1 0 1 1 1 0 X X X X X X X X X X X X X X X X l Another sub block of addresses could be as follows 1 0 0 0 0 0 0 0 1 0 1 0 1 1 1 0 1 0 0 0 1 1 0 1 X X X X X X X X l l l l First 24 bits identify the network and the subnetwork The last 8 bits identify the host within the subnetwork This subnetwork is identified as 128 174 141 0 zero for the host bits It also has associated with it a subnet mask 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 1 network or subnet bit 0 host bit 8 Why a subnet mask l Assume I tell you about a subnet number 128 174 141 0 l How many bits are for the network subnet and how many bits are for the host Can you tell l Routing table entries within the organization include the subnet number AND its associated subnet mask 9 Subnetting in summary l Assumptions Subnets are close together same company l Looks like a single network to routers outside the organization Hence outside routers only have a single entry in their table for the organization IP with Subnetting All hosts in the same company have the same network All hosts on the same physical network must have the same subnet an extension of network 10 Subnetting Example Subnet Mask 255 255 255 128 last byte bin 1000 0000 Subnet Number 128 174 142 128 last byte bin 1000 0000 H1 128 174 142 200 R1 Subnet Mask 255 255 255 128 last byte bin 1000 0000 Subnet Number 128 174 142 0 last byte bin 0000 0000 128 174 141 3 H3 R2 H2 128 174 142 27 Subnet Mask 255 255 255 0 Subnet Number 128 174 141 0 Note subnet masks are not of the same length Can we replace the subnet mask of 128 174 142 0 by 255 255 255 0 Can we have an additional subnet 128 174 142 0 with mask 255 255 255 0 Can we have an additional subnet 128 174 140 0 How many bits can we have in the mask 11 Subnetting host IP AND SM SN Host 3 128 174 141 3 1 0 0 0 0 0 0 0 1 0 1 0 1 1 1 0 1 0 0 0 1 1 0 1 0 0 0 0 0 0 1 1 Subnet Mask 255 255 255 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 Subnet 128 174 141 0 1 0 0 0 0 0 0 0 1 0 1 0 1 1 1 0 1 0 0 0 1 1 0 1 0 0 0 0 0 0 0 0 Host 1 128 174 142 200 1 0 0 0 0 0 0 0 1 0 1 0 1 1 1 0 1 0 0 0 1 1 1 0 1 1 0 0 1 0 0 0 Subnet Mask 255 255 255 128 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 …
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