Internet Networking
Spring 2002
Tutorial 1
Subnets, Proxy ARP
Slides of the course was made by TAs of
this and previous semesters
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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)
We study in this course
Network (IP)
Link
Physical
Introduction to Networking
Electronic Engineering
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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
Network 128.10.1.0
128.10.1.1
H1
Rest of the
Internet
128.10.1.2
H2
R
.
Network 128.10.2.0
All traffic to
128.10.0.0
H3
128.10.2.1
H4
128.10.2.2
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
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.
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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.
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