Download Scaling the Network: Subnetting and Other Protocols Networking

Scaling the Network:
Subnetting and Other Protocols
Networking
CS 3470, Section 1
Today
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
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CIDR
Subnetting
Private IP addresses
ICMP, IMAP, and DHCP Protocols
2
Packet Encapsulation
** Creative Commons: http://en.wikipedia.org/wiki/File:UDP_encapsulation.svg
3
IP Addressing

Classful addressing scheme separates groups of
addresses into classes

Class A
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
Class B
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
8 bits used for network (256)
24 bits used for hosts and network devices (16,777,216)
Binary address starts with 0
16 bits for networks (65,536)
16 bits for hosts and network devices (65,536)
binary address starts with 10
Class C
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24 bits for the network (16,777,216)
8 bits for the host (256)
Binary address starts with 110
Classless Inter-Domain Routing
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Classful addressing scheme wasteful
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IP address space exhaustion
Class B net allocated enough for 65K hosts
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Even if only 2K hosts in that network
Solution: Classless Inter Domain Routing
(CIDR)
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Eliminate class distinction
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No A,B,C
Keep multicast class D
5
Classless Addressing
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Addresses allocated in contiguous blocks
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Number of addresses assigned always power of 2
Network portion of address is of arbitrary length
Address format: a.b.c.d/x
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x is number of bits in network portion of address
network
part
host
part
11001000 00010111 1
0000000 00000000
200.23.128.0/17
6
Subnet Motivation
network
part
host
part
11001000 00010111 1
0000000 00000000
200.23.128.0/17

This network can have 215 = 32,768 hosts!


Imagine the size of the routing tables if we had a
flat network of all these hosts!
We want to split this network up into smaller
networks
7
Subnet Motivation
network
part
host
part
11001000 00010111 1
0000000 00000000
200.23.128.0/17

We probably want to split this network up into
smaller networks (subnets) due to


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Security reasons
Logistical reasons
Routing reasons
8
Let’s play with a small example
network
part
host
part
11000000 10101000 00001010 00000000
192.168.10.0/24

Suppose you have this private class C
network, and you need to divide it evenly

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You will have hosts 0-127
Friend will have hosts 128-255
9
Let’s play with a small example
network
part
host
part
11000000 10101000 00001010 00000000
192.168.10.0/24

Dividing the network into subnets involves
using some of the host bits as the subnet ID

What bit of the host part of the address do we
have to flip to signify >= 128 for the host ID?
10
Let’s play with a small example
network
part
subnet
ID
host
part
0
0000000
subnet
ID
host
part
1
0000000
11000000 10101000 00001010
192.168.10.0/25

Can address hosts 0-127
network
part
11000000 10101000 00001010
192.168.10.128/25

Can address hosts 128-255
11
Let’s play with a small example
network
part
11000000 10101000 00001010
subnet
ID
host
part
0
0000000
192.168.10.0/25

Now, how can routers easily figure out where
destination IP address 192.168.10.202
should be routed?
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192.168.10.0/25 or 192.168.10.128/25 subnet?
12
Let’s play with a small example
network
part
11000000 10101000 00001010
subnet
ID
host
part
0
0000000
192.168.10.0/25
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A subnet number is the network part + subnet
ID + zeros for the host
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192.168.10.0
A subnet mask consists of all 1’s for the
network+subnet ID and all 0’s for the host part
What is this subnet mask?
13
Let’s play with a small example
network
part
11000000 10101000 00001010
subnet
ID
host
part
0
0000000
192.168.10.0/25

Subnet mask:
255.255.255.128
11111111 11111111 11111111
10000000
14
Let’s play with a small example
network
part
11000000 10101000 00001010
subnet
ID
host
part
1
0000000
192.168.10.128/25

Subnet mask:
255.255.255.128
11111111 11111111 11111111
10000000
15
Subnet Masks
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We can figure out where to route by noting
that
dest subnet = subnet mask & dest IP addr
16
Subnet Masks
dest subnet = subnet mask & dest IP addr
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Let’s say destination IP is 192.168.10.202
and lets & with subnet mask
11000000
11111111
11000000
10101000
11111111
10101000
00001010
11111111
00001010
110001010
100000000
100000000
&
192 . 168 . 10
. 128
We send packet to 192.168.10.128/25 network!
17
Longest-Prefix Match
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Suppose two network IDs exist:
1) 128.186.0.0/16
2) 128.186.134.0/24
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Suppose you have destination IP of
128.186.134.100
Both subnet mask & IP of 1 and 2 will yield
match – what to do?
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Longest-prefix match – route to network with
the most matching host bits.
18
Subnetting
Notes
 Would use a default router if nothing matches
 Not necessary for all ones in subnet mask to be
contiguous
 Can put multiple subnets on one physical network
 Subnets not visible from the rest of the Internet
Routing with CIDR
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Destination IP is BA.DB.EE.F2
20
Special IP Addresses
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Network address: host id = all 0’s
Local broadcast address: all 1’s
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Directed broadcast address: host id = all 1’s
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Routers will forward this broadcast address
Local host address (this computer): all 0’s
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Used during system startup
Used during system startup
Loopback address
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network id = 127, any host id (e.g. 127.0.0.1)
21
Private IP Addresses
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Some addresses are not globally routable
IP packets created by these addresses
cannot be transmitted into the public domain
Commonly used for home, office, and
enterprise LANS
22
Private IP Addresses
Address Range
CIDR
Number of
Addresses
10.0.0.0 – 10.255.255.255
10.0.0.0/8
16,777,216
172.16.0.0 – 172.31.255.255
172.16.0.0/12
1,048,576
192.168.0.0 – 192.168.255.255
192.168.0.0/16
65,535
23
Private IP addresses

Router uses Network Address Translation (NAT) to
send IP packets from private IP addresses onto
public networks
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Router places it’s own IP address as destination
Maintains table, knows which host to route addresses
Router keeps
translation table
24
IP Address Configuration
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May configure a network statically by giving
each host it’s IP address and routing
information (like gateway)
Or may configure a server to do this for you
dynamically
25
DHCP Server
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Dynamic Host Configuration Protocol (DHCP)
DHCP server is responsible for providing
configuration information to hosts
There is at least one DHCP server for an
administrative domain
DHCP server maintains a pool of available
addresses
DHCP Protocol
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State Protocol
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DHCPDISCOVER (client)
DHCPOFFER (server)
DHCPREQUEST (client)
DHCPACK (server)
DHCPNAK (server)
DHCPINFORM (client)
DHCP
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Newly booted or attached host sends
DHCPDISCOVER message to a special IP address
(255.255.255.255)
Rest of messages are unicast back and forth
DHCP
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IP leases are valid for a predefined period of
time (T1)
Leases are renewed at T1/2
Leases are released if they have not been
renewed at the expiration of the lease time
Internet Control Message Protocol
(ICMP)

Defines a collection of error messages that
are sent back to the source host whenever a
router or host is unable to process an IP
datagram successfully

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Destination host unreachable due to link /node
failure
Reassembly process failed
TTL had reached 0 (so datagrams don't cycle
forever)
IP header checksum failed