Internet Networking recitation #1

needleorderNetworking and Communications

Oct 24, 2013 (3 years and 7 months ago)

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1
Winter Semester 2011, Dept. of Computer Science, Technion
Internet Networking
recitation #1
Subnet + CIDR
2
Internet Networking
Administrative Information
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Course site:
webcourse.cs.technion.ac.il/236341
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Grading policy:
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15% homeworks+ 85% final exam
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6 home assignments.
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Submission is in pairs.
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Teaching Assistant:
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Ella Bolshinsky

Phone:(829)3939

Office location:Taub315

Teaching hours: Monday 17:30 -18:30
Wednesday 9:30 -10:30

Reception hour: Wednesday 10:30 -11:30
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Internet Networking
IP Addressing: Original ClassfulScheme
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IP Address 32-bit integer globally unique address
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Dotted Notation: 132.68.37.54
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IP Classes dividing an address to net idand host id
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The prefix (net id) identifies a network.
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The suffix (host id) identifies a host on this network.
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Internet Networking
IP Addressing: Original ClassfulScheme
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Class A 7 bits to net id, 24 bits to host id 1.0.0.0 126.0.0.0
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Class B 14 bits to net id, 16 bits to host id 128.0.0.0 191.255.0.0
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Class C 21 bits to net id, 8 bits to host id 192.0.0.0 223.255.255.0
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Class D for multicasting
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Class E reserved for future use (used for private addresses)
Weakness
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Growth of routing tables in routers
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Tens of thousands small (class C) networks.
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Each network must be advertised.
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Inflexible
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Lack of a network classes for mid-sized organization (between
class B and C).
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Address space will be eventually exhausted
5
Internet Networking
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.
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Limits of technologies.
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Network congestion.
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Security consideration.

VLAN separate one physical network into a few logical
networks.
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Administration (e.g. different departments in academic institute).
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From outside it looks like a single network
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Only local routers know about multiple physical networks inside and
how to route traffic among them
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Host ID is divided into a subnet ID and host ID
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Accepted as a standard at 1985 (RFC 950).
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Internet Networking
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 contains a net mask.
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Routing is done on a longest-match basis.
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If the packet is destined to other network then the router
sends it to another router.
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Otherwise the router sends the packet to the appropriate
host on its attached networks.
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Internet Networking
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
128.10.2.1128.10.2.2
All traffic to
128.10.0.0/16
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A site with two physical networks.
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Using subnetting, R advertise these networks as a single network (thus,
R accepts all traffic for net 128.10.0.0)
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Internal routing is done according to subnet id (i.e. the third octet of the
address).
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Internet Networking
Variable-Length Subnetting
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Motivation: Consider the case when an organization
has a few networks of different sizes.
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When we choose the subnet partitioning, we actually
define constant number of possible physical
subnetworkswith maximum number of hosts on them.
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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.
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Solution: Variable-Length Subnetting. A subnet
partition is selected on a per-network basis.
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Internet Networking
Configuring a Network with Variable-Length Subnetting
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We have a network with IP 202.128.236.0/24
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We need to support next sub networks:
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6 networks with 26 hosts
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3 networks with 10 hosts
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4 networks with 2 hosts
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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
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We need only 6 such sub networks.
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The rest 2 sub networks we will partition by subnet mask of /28 bits.
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11111111.11111111.11111111.11110000
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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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Internet Networking
Configuring a Network with Variable-Length Subnetting
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The rest we will partition by subnet mask of /30 bits.
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11111111.11111111.11111111.11111100
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We will get 4 sub networks of 2 hosts in each.
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Subnet mask #1 = /27
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11111111.11111111.11111111.11100000
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Subnet mask #2 = /28
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11111111.11111111.11111111.11110000
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Subnet mask #3 = /30
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11111111.11111111.11111111.11111100
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Internet Networking
Classless Inter-Domain Routing (CIDR) -RFC 1519
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Routing destinations are represented by network and mask pairs.
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Enabling network aggregation; thereby reducing the size of
routing table.
Examples:
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Class A networks are followed by a /8
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Class C networks are followed by a /24
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8 Class C hosts network is followed by /21
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Such a network has 21 bits of Net-ID, 11 Bits of Host-ID
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Contains 2^
21
Net IDs, and 2^
11
-2 = Hosts in Each network.