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Internet Networking
Spring 2004
Tutorial 1
Subnetting and CIDR
Proxy ARP
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Administrative Information
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Course site:
webcourse.technion.ac.il/236341
Assistants:
Rami Cohen, Taub 524, (829)4899
email: [email protected]
Galperin Vadim, Taub 441, (829)3941
email: [email protected]
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Reminding – IP Addressing
(Original Classful Scheme)
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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
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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)
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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
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Growth of routing tables in routers
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Tens of thousands small (class C)
networks.
Each network must be advertised.
Inflexible
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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)
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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:
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Flexibility
Enabling network aggregation; thereby reducing
the size of routing table.
Defined in RFC 1519.
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Subnet Addressing
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A site has a single IP network address
assigned to it, but has two or more physical
networks.
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Different technologies.
Limits of technologies.
Network congestion.
Security consideration.
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VLAN – separate one physical network into a few logical
networks.
Administration (e.g. deferent departments in
academic institute).
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Subnet Addressing
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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
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When a router gets a packet, it isolates by
Net mask the packet net id address.
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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
Network 128.10.1.0/24
128.10.1.1
Rest of the
Internet
R
H1
128.10.1.2
H2
.
Network 128.10.2.0/24
All traffic to
128.10.0.0/16
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H3
128.10.2.1
H4
128.10.2.2
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
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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
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We have a network with IP
202.128.236.0/24
We need to support next sub networks:
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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
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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).
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11111111.11111111.11111111.11100000
We need only 6 such sub networks.
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Example – Configuring a Network
with Variable-Length Subnetting
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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
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We need only 3 such sub networks.
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Example – Configuring a Network
with Variable-Length Subnetting
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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
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Subnet mask #1 = 202.128.236.0 /27
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Subnet mask #2 = 202.128.236.0 /28
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11001010.10000000.11101100.11100000
11001010.10000000.11101100.11110000
Subnet mask #3 = 202.128.236.0 /30
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11001010.10000000.11101100.11111100
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Reminding - ARP
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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
ARP Response (00:11:22:33:44:55:12)
ARP Request (10.4.10.90)
10.4.10.90
10.4.10.91
10.4.10.86
10.4.10.20
10.4.10.23
00:11:22:33:44:55:12
00:11:22:33:44:55:bc
00:11:22:33:44:55:2e
00:11:22:33:44:55:6b
00:11:22:33:44:55:40
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Proxy ARP
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Proxy ARP (also called promiscuous ARP or
ARP hack) is a technique used to map a
single IP network prefix into two physical
addresses.
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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
Network A: 10.10.1.x/24
10.10.1.3
10.10.1.4
R
PPP
.
Network B:
10.10.1.x/24
10.10.1.5
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Proxy ARP
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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.
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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.
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