Slides of the course was made by TAs of this and previous semesters

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Internet Networking Spring 2002

Tutorial 1

Subnets, Proxy ARP

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Administrative Information

• Course site: webcourse.technion.ac.il/236341

• Assistants: Getselevich Vladimir, Taub 327, (829)4309

email: vladg@tx.tecnion.ac.il Kapchits Boris, Taub 325, (829)4307

email: boria@tx.tecnion.ac.il

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Reminding - TCP/IP Layers

Application (HTTP)

Transport (TCP,UDP)

Network (IP)

Link

Physical Electronic Engineering

Introduction to Networking

We study in this course

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Reminding – IP Addressing (Original Classful Scheme)

• IP Address – 32-bit integer globally unique address

• Dotted Notation: 132.68.37.54

• IP Classes – dividing an address to net id and host id prefix of an IP address (net id) identifies a network and a suffix (host id) identifies a host on this network

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Reminding – IP Addressing (Original Classful Scheme)

• Class A – 7 bits to net id, 24 bits to host id 1.0.0.0 – 126.0.0.0

• Class B – 14 bits to net id, 16 bits to host id 128.1.0.0 – 191.255.0.0

• Class C – 21 bits to net id, 8 bits to host id 192.0.1.0 – 223.255.255.0

• Class D – for multicasting

• Class E – reserved for future use

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Weaknesses of Classful Scheme

Growth!!! Tens of thousands small networks.

• Extremely large routing tables• Address space will be eventually exhausted• Complex administration

How can one minimize the number of assigned network addresses, especially class B, without abandoning 32-bit addressing scheme?

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Subnet Addressing

• A site has a single IP network address assigned to it, but has two or more physical networks

• From outside it looks like a single network• Only local routers know about multiple physical

networks inside and how to route traffic among them

• Host ID is divided into a subnet ID and host ID

Accepted as a standard by RFC 950, 1985

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How a Router Perform Routing

Usual Routing• When a router get a packet, it isolates by Net

mask the packet net id address - if the packet is destined to other network then the router sends it to another router; otherwise according to host id, the router sends the packet to the appropriate host on its network.

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How a Router Perform Routing

Routing with subnetting• When a router get a packet, it isolates by Net

mask the packet net id address - if the packet is destined to other network then the router sends it to another router; otherwise the router isolates by Subnet mask at subnet id address of the packet – if it destined to another sub network then it sent to another internal router; otherwise according to host id, the router sends the packet to the appropriate host on its network.

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Subnetting - Example

R

.

Network 128.10.1.0

H1 H2

Network 128.10.2.0

H3 H4

128.10.1.1

Rest of the Internet

128.10.1.2

128.10.2.1 128.10.2.2All traffic to 128.10.0.0

A site with two physical networks using subnet addressing to label them with a single class B network address. Router R accepts all traffic for net 128.10.0.0 and chooses a physical network based on the third octet of the address.

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Subnet Addressing

• Subnetting is hierarchical addressing scheme and it accommodates large growth because a given router doesn’t need to know as much detail about distant destinations as it does about local ones.

• It’s up to local policy to decide how to partition the local part of the IP address between subnet id and host id. When there is a compromise between large number of subnets with small number of hosts and the opposite.

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Variable-Length Subnetting

• When we choose the subnet partitioning, we actually define constant number of possible physical subnetworks with maximum number of hosts on them.

• Difficult to keep small and big subnetworks and there could be unnecessary spending of address space.

• Solution is Variable-Length Subnetting – when a subnet partition is selected on a per-network basis.

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Example – Configuring a Network with Variable-Length Subnetting

• We have a network with IP 202.128.236.0

• We need to support next sub networks:– 6 networks with 26 hosts– 3 networks with 10 hosts– 4 networks with 2 hosts

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Example – Configuring a Network with Variable-Length Subnetting

• The given network is of Class C• Its Net Mask is: 255.255.255.0 (the network id is

24 bits and local part is 8 bits)• If we take subnet mask of /27 bits then we can

get 8 sub networks of 30 hosts (all 0’s and all 1’s of host addresses are reserved).– 11111111.11111111.11111111.11100000

We need only 6 such sub networks

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Example – Configuring a Network with Variable-Length Subnetting

• The rest 2 sub networks we will partition by subnet mask of /28 bits.

• We will get 4 sub networks of 14 hosts in each– 11111111.11111111.11111111.11110000

We need only 3 such sub networks

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Example – Configuring a Network with Variable-Length Subnetting

• The rest we will partition by subnet mask of /30 bits.

• We will get 4 sub networks of 2 hosts in each– 11111111.11111111.11111111.11111100

and that is all what we needed!

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Example – Configuring a Network with Variable-Length Subnetting

• Subnet mask #1 = 202.128.236.0 /27

– 11001010.10000000.11101100.11100000

• Subnet mask #2 = 202.128.236.0 /28

– 11001010.10000000.11101100.11110000

• Subnet mask #2 = 202.128.236.0 /30

– 11001010.10000000.11101100.11111100

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Reminding - ARP

• ARP (Address Resolution Protocol) serves for mapping from high-level IP address into low level MAC address.

• Two machines on a given network can communicate only if they know each other’s physical network address

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Reminding - ARP

• When host A wants to resolve IP address Ib, it broadcasts a special packet that asks the host with IP address Ib to respond with its physical address, Pb. All hosts, including B, receive the request, but only host B recognizes its IP address and sends a reply that contains its physical address. When A receives the reply, it uses the physical address to send the internet packet directly to B.

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Proxy ARP

• Proxy ARP (also called promiscuous ARP or ARP hack) is a technique used to map a single IP network prefix into two physical addresses.

• Assume that there are 2 networks A and B connected by router R that runs Proxy ARP

• R knows IP addresses from both sides (knows where each host is located)

• R uses ARP to hide one of networks

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Proxy ARP

.

H1 H2

H4 H5

H3

Network A

Network B

R Router running proxy ARP

Router R answers ARP requests on each network for hosts on other network, giving its hardware address and then routing datagrams correctly when they arrive.

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Proxy ARP - Example

• Assume that host H1 from network A wants to send a packet to host H4 from network B.

– H1 sends ARP request to get MAC address of H4.– Router R will catch this ARP request– R knows that H4 is on network B answers with its own

MAC address– H1 will store this address in its cash and from now H1

will send to R packets which are destined to H4.– R according to its routing table will send the packets

to H4.

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ARP Features

• ARP is a trusted protocol – therefore proxy ARP can work

• Weakness of ARP is possibility of IP Spoofing– ARP implementations that warning alerts on possible

security violations will not work good with Proxy ARP

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Proxy ARP cons/pros

• The advantage of proxy ARP is that it can be added to a single router on a network without disturbing the routing tables in other hosts or router on that network.

• The disadvantage of proxy ARP is that it does not work for networks unless they use ARP for address resolution and it does not generalize to more complex network topology.