Scaling the Network: Subnetting and Other Protocols. Networking CS 3470, Section 1

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Scaling the Network:

Subnetting and Other Protocols

Networking

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Today

 CIDR

 Subnetting

 Private IP addresses

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IP Addressing

 Classful addressing scheme separates groups of

addresses into classes

 Class A

 8 bits used for network (256)

 24 bits used for hosts and network devices (16,777,216)  Binary address starts with 0

 Class B

 16 bits for networks (65,536)

 16 bits for hosts and network devices (65,536)  binary address starts with 10

 Class C

 24 bits for the network (16,777,216)  8 bits for the host (256)

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Classless Inter-Domain Routing

 Classful addressing scheme wasteful

 IP address space exhaustion

 Class B net allocated enough for 65K hosts

 Even if only 2K hosts in that network

 Solution: Classless Inter Domain Routing

(CIDR)

 Eliminate class distinction

 No A,B,C

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Classless Addressing

 Addresses allocated in contiguous blocks

 Number of addresses assigned always power of 2

 Network portion of address is of arbitrary length  Address format: a.b.c.d/x

 x is number of bits in network portion of address

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Subnet Motivation

 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

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Subnet Motivation

 We probably want to split this network up into

smaller networks (subnets) due to

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Let’s play with a small example

 Suppose you have this private class C

network, and you need to divide it evenly

 You will have hosts 0-127

 Friend will have hosts 128-255

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Let’s play with a small example

 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?

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Let’s play with a small example

 Can address hosts 0-127

 Can address hosts 128-255

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Let’s play with a small example

 Now, how can routers easily figure out where

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Let’s play with a small example

 A subnet number is the network part + subnet

ID + zeros for the host

 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?

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Subnet Masks

 We can figure out where to route by noting

that

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Subnet Masks

dest subnet = subnet mask & dest IP addr

 Let’s say destination IP is 192.168.10.202

and lets & with subnet mask

192 . 168 . 10 . 128

We send packet to 192.168.10.128/25 network!

11000000 10101000 00001010 110001010 &

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Longest-Prefix Match

 Suppose two network IDs exist:

1) 128.186.0.0/16 2) 128.186.134.0/24

 Suppose you have destination IP of

128.186.134.100

 Both subnet mask & IP of 1 and 2 will yield

match – what to do?

 Longest-prefix match – route to network with

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Subnetting

Notes

 Would use a default router if nothing matches  Not necessary for all ones in subnet mask to be

contiguous

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Routing with CIDR

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Special IP Addresses

 Network address: host id = all 0’s  Local broadcast address: all 1’s

 Used during system startup

 Directed broadcast address: host id = all 1’s

 Routers will forward this broadcast address

 Local host address (this computer): all 0’s

 Used during system startup

 Loopback address

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Private IP Addresses

 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

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Private IP Addresses

Address Range CIDR Number of

Addresses

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Private IP addresses

 Router uses Network Address Translation (NAT) to

send IP packets from private IP addresses onto public networks

 Router places it’s own IP address as destination

 Maintains table, knows which host to route addresses

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IP Address Configuration

 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

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DHCP Server

 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

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DHCP

 Newly booted or attached host sends

DHCPDISCOVER message to a special IP address (255.255.255.255)

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DHCP

 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

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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

 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