Scaling the Network Subnetting and Protocols Networking CS

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

Today ARP, IMAP, and DHCP Protocols Subnetting 2

Packet Encapsulation ** Creative Commons: http: //en. wikipedia. org/wiki/File: UDP_encapsulation. svg 3

Address Resolution IP address is virtual Not understood by underlying physical networks IP packets need to be transmitted by the underlying physical network Address resolution Translating IP address to physical address Address Resolution Protocol (ARP) 4

ARP A router has to know where to deliver packets on the local network ARP is used to discover MAC addresses based on IP addresses arp who-has 192. 168. 10. 1 tell node 31. ceee. lab arp reply 192. 168. 10. 1 is-at 00: 60: 08: ce: 9 d: 3 b arp who-has node 31. ceee. lab tell 192. 168. 10. 254 arp reply node 31. ceee. lab is-at 00: 02: 3 f: b 4: cd: 87

ARP Cache Each computer maintains a cache table IP address hardware address mapping Only about computers on the same network Try out “/usr/sbin/arp –a” command Exchanges ARP messages To resolve IP addresses with unknown hardware addresses Encapsulated in Ethernet data frame 6

ARP Protocol When a node sends an IP packet Look up destination address in the ARP table If not found To another node on the same physical network Broadcast a request to the local network Whose IP address is this? What info should the request message contain? 7

ARP Response The target node responds to sender (unicast? ) On receiving the response Requester updates its table Other nodes upon receiving the request With its physical address Adds the requester into its ARP table (why? ) Refresh the requester entry if already there No action otherwise (why? ) Table entries deleted if not refreshed for a while 8

ARP Example ARP broadcast by W requesting hardware address of Y 9

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 dynamically 10

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) Rest of messages are unicast back and forth

DHCP IP leases are valid for a predefined period of time (T 1) Leases are renewed at T 1/2 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

IP Addressing Classful addressing scheme separates groups of addresses into classes Class A Class B 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 65 K hosts Even if only 2 K hosts in that network Solution: Classless Inter Domain Routing (CIDR) Eliminate class distinction No A, B, C Keep multicast class D 17

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 00000000 200. 23. 128. 0/17 18

Subnet Motivation network part host part 11001000 00010111 1 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 19

Subnet Motivation network part host part 11001000 00010111 1 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 20

Let’s play with a small example network part host part 11000000 10101000 00001010 0000 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 21

Let’s play with a small example network part host part 11000000 10101000 00001010 0000 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? 22

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 23

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? 192. 168. 10. 0/25 or 192. 168. 10. 128/25 subnet? 24

Let’s 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 1’s for the network+subnet ID and all 0’s for the host part What is this subnet mask? 25

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. 128 11111111 10000000 26

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. 128 11111111 10000000 27

Subnet Masks We can figure out where to route by noting that dest subnet = subnet mask & dest IP addr 28

Subnet Masks dest subnet = subnet mask & dest IP addr Let’s say destination IP is 192. 168. 10. 202 and lets & with subnet mask 11000000 1111 11000000 10101000 1111 10101000 00001010 1111 00001010 110001010 100000000 & 192. 168. 10. 128 We send packet to 192. 168. 10. 128/25 network! 29

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. 30

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 Destination IP is BA. DB. EE. F 2 32