Internet Networking Spring 2002

1. Internet Networking Spring 2002 Tutorial 1 Subnets, Proxy ARP Slides of the course was made by TAs of this and previous semesters

2. 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

3. Reminding - TCP/IP Layers We study in this course Introduction to Networking Electronic Engineering

4. 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

5. 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

6. 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?

7. 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

8. 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.

9. 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.

10. Subnetting - Example Network 128.10.1.0 128.10.1.1 128.10.1.2 H1 H2 Rest of the Internet R . Network 128.10.2.0 All traffic to 128.10.0.0 128.10.2.1 128.10.2.2 H3 H4 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.

11. 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.

12. 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.

13. Example – Configuring a Network withVariable-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

14. Example – Configuring a Network withVariable-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

15. Example – Configuring a Network withVariable-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

16. Example – Configuring a Network withVariable-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!

17. Example – Configuring a Network withVariable-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

18. 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

19. 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.

20. 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

21. Proxy ARP Network A H1 H2 H3 R Router running proxy ARP H4 H5 . Network B 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.

22. Proxy ARP - Example • Assume that host H1from network A wants to send a packet to host H4from network B. • H1sends 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 H1will send to R packets which are destined to H4. • R according to its routing table will send the packets to H4.

23. 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

24. 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.