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

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Internet Networking Spring 2004

Tutorial 1 Subnetting and CIDR

Proxy ARP

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Administrative Information Course site: webcourse.technion.ac.il/236341

Assistants: Rami Cohen, Taub 524, (829)4899

email: ramic@cs.tecnion.ac.il Galperin Vadim, Taub 441, (829)3941

email: gvadim@cs.tecnion.ac.il

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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 The prefix (net id) identifies a network. The 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 (used

for private addresses)

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Weaknesses of Classful Scheme

Growth of routing tables in routers Tens of thousands small (class C)

networks. Each network must be advertised.

Inflexible Lack of a network classes for mid-sized

organization (between class B and C). Address space will be eventually

exhausted

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Classless Inter-Domain Routing (CIDR) The concept of network "class" is

deprecated. Routing destinations are represented by

network and mask pairs. Routing is performed on a longest-

match basis Advantages:

Flexibility Enabling network aggregation; thereby

reducing the size of routing table. Defined in RFC 1519.

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Subnet Addressing A site has a single IP network address

assigned to it, but has two or more physical networks. Different technologies. Limits of technologies. Network congestion. Security consideration.

VLAN – separate one physical network into a few logical networks.

Administration (e.g. deferent departments in academic institute).

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Subnet Addressing 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 at 1985 (RFC 950).

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Subnet Routing When a router gets a packet, it isolates

by Net mask the packet net id address. Each routing entry contain a net mask. Routing is done on a longest-match basis.

If the packet is destined to other network then the router sends it to another router.

Otherwise the router sends the packet to the appropriate host on its attached networks.

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Subnetting - Example

R

.

Network 128.10.1.0/24

H1 H2

Network 128.10.2.0/24

H3 H4

128.10.1.1

Rest of the Internet

128.10.1.2

128.10.2.1 128.10.2.2All traffic to 128.10.0.0/16

A site with two physical networks. Using subnetting, R advertise these networks as a single

network (thus, R accepts all traffic for net 128.10.0.0) Internal routing is done according to subnet id (i.e. the

third octet of the address).

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Variable-Length Subnetting Motivation: Consider the case when an

organization has a few network of different sizes.

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 (waist of subnet numbers) and big (the host id needs more bits) sub networks and there could be unnecessary spending of address space.

Solution: Variable-Length Subnetting. 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 202.128.236.0/24

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

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). 11111111.11111111.11111111.1110

0000 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 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.

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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 #3 = 202.128.236.0 /30 11001010.10000000.11101100.11111100

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Reminding - ARP Two machines on a given network can

communicate only if they know each other’s physical network address

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

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Reminding – ARP

10.4.10.91

00:11:22:33:44:55:bc

ARP Request (10.4.10.90)

ARP Response (00:11:22:33:44:55:12)

10.4.10.90

00:11:22:33:44:55:12

10.4.10.20

00:11:22:33:44:55:6b

10.4.10.23

00:11:22:33:44:55:40

10.4.10.86

00:11:22:33:44:55:2e

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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. Using the same network address space for

more than one physical address Assume that there are 2 networks A and

B connected by router R that runs Proxy ARP

Using Proxy ARP, R can use the same net id for both networks.

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Proxy ARP

.

R

PPP

Network A: 10.10.1.x/24

Network B:

10.10.1.x/24

10.10.1.5

10.10.1.3 10.10.1.4

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Proxy ARP Router R replies to ARP requests that are

generated by hosts on the PPP connection (Network B), in which the target IP is on network A, namely it sends its MAC address. R know which hosts are connected through the

PPP. These host assume that the destination

host are on the same physical network. In their ARP table the router MAC address is

associated with the destination IP address.