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

Transcription

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

2 Today CIDR Subnetting Private IP addresses ICMP, IMAP, and DHCP Protocols 2

3 Packet Encapsulation ** Creative Commons: 3

4 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) Binary address starts with 110

5 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

6 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 /17 host 6

7 Subnet Motivation network /17 host This network can have 2 15 = 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

8 Subnet Motivation network /17 host We probably want to split this network up into smaller networks (subnets) due to Security reasons Logistical reasons Routing reasons 8

9 Let s play with a small example network /24 Suppose you have this private class C network, and you need to divide it evenly You will have hosts Friend will have hosts host 9

10 Let s play with a small example Dividing the network into subnets involves using some of the host bits as the subnet ID network /24 host What bit of the host of the address do we have to flip to signify >= 128 for the host ID? 10

11 Let s play with a small example network /25 Can address hosts network Can address hosts subnet ID host /25 subnet ID host 11

12 Let s play with a small example Now, how can routers easily figure out where destination IP address should be routed? network /25 subnet ID host /25 or /25 subnet? 12

13 Let s play with a small example network /25 A subnet number is the network + subnet ID + zeros for the host subnet ID host A subnet mask consists of all 1 s for the network+subnet ID and all 0 s for the host What is this subnet mask? 13

14 Let s play with a small example network /25 Subnet mask: subnet ID host 14

15 Let s play with a small example network /25 Subnet mask: subnet ID host 15

16 Subnet Masks We can figure out where to route by noting that dest subnet = subnet mask & dest IP addr 16

17 Subnet Masks dest subnet = subnet mask & dest IP addr Let s say destination IP is and lets & with subnet mask & We send packet to /25 network! 17

18 Longest-Prefix Match Suppose two network IDs exist: 1) /16 2) /24 Suppose you have destination IP of 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

19 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

20 Routing with CIDR Destination IP is BA.DB.EE.F2 20

21 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 network id = 127, any host id (e.g ) 21

22 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

23 Private IP Addresses Address Range CIDR Number of Addresses /8 16,777, /12 1,048, /16 65,535 23

24 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 Router keeps translation table 24

25 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 25

26 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

27 DHCP Protocol State Protocol DHCPDISCOVER (client) DHCPOFFER (server) DHCPREQUEST (client) DHCPACK (server) DHCPNAK (server) DHCPINFORM (client)

28 DHCP Newly booted or attached host sends DHCPDISCOVER message to a special IP address ( ) Rest of messages are unicast back and forth

29 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

30 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