# Part I: Introduction

Scaling the Network: Subnetting and Other Protocols Networking CS 3470, Section 1 Today CIDR Subnetting Private IP addresses ICMP, IMAP, and DHCP Protocols 2 Packet Encapsulation ** Creative Commons: http://en.wikipedia.org/wiki/File:UDP_encapsulation.svg

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 24 bits for the network (16,777,216) 8 bits for the host (256) Binary address starts with 110 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

Keep multicast class D 5 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 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 Security reasons Logistical reasons Routing reasons 8 Lets 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 You will have hosts 0-127 Friend will have hosts 128-255 9 Lets 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 Lets 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 Lets 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? 192.168.10.0/25 or 192.168.10.128/25 subnet? 12

Lets play with a small example network part 11000000 10101000 00001010 subnet ID host part 0 0000000 192.168.10.0/25 A subnet number is the network part + subnet

ID + zeros for the host 192.168.10.0 A subnet mask consists of all 1s for the network+subnet ID and all 0s for the host part What is this subnet mask? 13 Lets 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 Lets 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 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 Lets 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

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 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 Lets look at an example from homework 5 20

Used during system startup Used during system startup Loopback address network id = 127, any host id (e.g. 127.0.0.1) 21 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 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 Router places its own IP address as destination Maintains table, knows which host to route addresses

Router keeps translation table 24 IP Address Configuration May configure a network statically by giving each host its IP address and routing information (like gateway) Or may configure a server to do this for you dynamically 25 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 addresses DHCP Protocol State Protocol

DHCPDISCOVER (client) DHCPOFFER (server) DHCPREQUEST (client) DHCPACK (server) DHCPNAK (server) DHCPINFORM (client) DHCP 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

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

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 The ping application is a very common ICMPmessage-generator

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