CIT 384: Network AdministrationSlide #1 CIT 384: Network Administration IP.

CIT 384: Network Administration Slide #1

CIT 384: Network Administration

IP


Topics

1. Network Layer

2. IP Packets

3. IP Addresses

4. Subnets

5. IP Routing

6. ARP and DHCP


Routing

• The primary function of the network layer is routing.

• IP is the widely used network layer protocol.

• IP routing relies on the structure of IP addresses.


Routing Logic

PC1: If host on same Ethernet, send; otherwise, send to gateway router.

R1, R2: Lookup destination in route table and forward packet to specified router.

R3: Lookup destination in route table and send to local Ethernet.


Network Layer Interaction with Data Link Layer

PC1: Network layer encapsulates data in Ethernet frame.

R1: Extract IP packet from Ethernet frame, and encapsulate in HDLC frame to send to R2.

R2: Extract IP packet from HDLC frame and encapsulate in FR frame.

R3: Extract IP packet from FR frame and encapsulate in Ethernet frame.


IP Packet Header


IP Header

Protocol version: IPv4

Header length: 5-60 32-bit words

Type of service (TOS):– 3-bit precedence (ignored today)– 4 TOS bits (min delay (telnet), max throughput

(ftp), max reliability, min monetary cost)– unused 0 bit


IP Header

Total length: length of IP datagram (bytes)– maximum size: 65535 bytes– large packets fragmented at data link layer.– small packets may be padded to minimum length.

TTL: upper limit on number of router hops.Protocol: which protocol supplied packet data.Header checksum: IP header checksum


IP Fragments

IP packets may be fragmented by routers for transmission across different media.– Max IP packet size: 65536– Max Ethernet packet size: 1500

IP headers contain fragment data:– Don’t Fragment Flag: 0=allowed, 1=don’t– More Fragments Flag: 0=last, 1=more fragments– Identification: identifies single packet for

reassembly.– Fragment Offset: where contents of fragment go.


IP Addresses

32-bit integersOne for each network interface.Dotted decimal notation: ii.jj.kk.ll

172 . 16 . 254 . 1

10101100 00010000 11111110 00000001

1 byte

32 bits = 4 bytes


Grouping IP Addresses

Groups of consecutive IP addrs are called networks.

Routing table would only need 3 entries below.


Network and Host Parts

IP addresses are divided into two parts– Network ID (like zip code)– Host ID (like street address)

Network ID Host ID

Two special IP addresses– Network address (e.g. 130.4.0.0)– Broadcast address (e.g. 130.4.255.255)


Address ClassesClass A: 0.0.0.0-127.255.255.255

8-bit net ID, 24-bit host ID224 – 2 hosts per network; 126 networks

Class B: 128.0.0.0-191.255.255.25516-bit net ID, 16-bit host ID216 – 2 hosts per network; 16,384 networks

Class C: 192.0.0.0-223.255.255.25524-bit net ID, 8-bit host ID(28 – 2) = 254 hosts per network; 2,097,152 networks

Class D: 224.0.0.0-239.255.255.25528-bit multicast group ID

Class E: 240.0.0.0-255.255.255.255Reserved for future use



Assigning Addresses

ICANN assigns network numbers.– Internet Corporation for Assigned Network

Numbers.– ICANN gives authority to regional orgs, e.g.

ARIN (American Registry for Internet Numbers)– Typically to ISPs, universities, corporations.

ISP assigns IP addresses within network


Addressing without Subnetting


Why do we need subnets?

• Non-subnet design requires– 6 class B address spaces.– Each class B has 65,534 IP addresses

• Even if we assume many more PCs exist, some networks are drastically underutilized.– 150.6 only has 2 IP addresses (routers)


Addressing with Subnets


IP Addresses with Subnets

Route on network + subnet part of address.


IP Routing


Host Routing

1. If destination IP address on same subnet, send the packet directly to dest.

2. Otherwise, send packet to default gateway router.


Router Logic

1. Check data-link FCS field for errors. Discard packet if an error detected.

2. Extract IP packet and discard data link header and trailer.

3. Compare packet’s destination IP address with routing table, and find route that matches the address. This route identifies the outgoing interface of router.

4. Encapsulate IP packet inside a new data link header and trailer appropriate for outgoing interface, and forward the frame.


Routing Example

PC1 sending a packet to PC2.1. PC1 to R1.

2. R1 to R2.

3. R2 to R3.

4. R3 to PC2.


Routing Example


Routing Table

Where to send an IP packet to?Use a table lookup: routing tableSearch Process:

1. Search for a matching host address.2. Search for a matching network address.3. Search for a default route.

No route to destination: Host or network unreachable error if search fails.


Routing Tablest361m13 (10.1.0.90) > netstat –rn

Routing Table: IPv4Destination Gateway Flags Ref Use Int------------- -------------------- ----- -----10.1.0.0 10.1.0.90 U 1 4977 hme0224.0.0.0 10.1.0.90 U 1 0 hme0default 10.1.0.1 UG 1 66480 127.0.0.1 127.0.0.1 UH 6 798905 lo0


Routing Table

Destination: final destination host/networkGateway: next host in route to destinationFlags

U: Route is upG: Route is to a gateway (router)H: Route destination is a host (not a network)D: Route created by a redirectM: Route modified by a redirect


Routing Table

10.1.0.0direct access to local subnet

224.0.0.0multicast route

defaultforward packets to router at IP 10.1.0.1

127.0.0.1loopback


Routing Table SourcesManual (static) routes

Added with the route command.

ICMP redirects can alter routesRouter sends ICMP redirect when packet should’ve been

sent to another router.

Routing protocolsRouters exchange routes with each other using special

routing protocols.

Full internet router tables contain ~30,000 routes.

Source routingSender includes routing info in packet header.


IP Routing Protocol Goals

1. Dynamically learn and fill routing table with a route to all subnets in network.

2. If more than one route available, place the best route in the routing table.

3. Replace lost routes with the best available route as soon as possible.

4. Prevent routing loops.


Routing Protocol Procedure

1. Add a route to table for each directly connected subnet.

2. Tell neighbors about all routes in its routing table, including both directly connected routes and routes learned from other routers.

3. After learning a new route from neighbor, add that route to the table.


Routing Protocol Example

1. R3 learns directly connected subnet 150.150.4.0 to E0.

2. R3 sends update to R2, so R2 adds.

3. R2 sends update to R1, so R1 adds route for 150.150.4.0 with outgoing interface of S0, the interface it learned from.


Address Translation: DNS and ARP

Hannah wants to send packet to Jessie, but doesn’t know destination IP or MAC addrs.

Solution: address translation Jessie to IP address via DNS IP address to MAC address via ARP


DNS

Domain Name ServiceTranslates names to IP addresses and vice-versa.


ARP

Address Resolution Protocol1. Broadcast request for MAC address for dest IP.

2. Owner of dest IP replies with MAC address.

3. Cache address so don’t have to ARP for every packet.


ARP Example

sftp zappa.nku.edu

1. Obtains IP address from DNS.2. sftp asks TCP to connect to IP address3. TCP sends connection request to zappa using an IP

datagram4. Sending host emits ARP broadcast, asking for MAC

address of given IP address5. Destination host’s ARP layer receives broadcast, answers

with an ARP reply w/ IP->MAC mapping6. Sending host constructs Ethernet frame with destination

MAC address containing IP datagram7. Sending host sends IP datagram


ARP Cache

st361m13 (10.1.0.90) > arp -a

Net to Media Table: IPv4Device IP Address Phys Addr ------ -------------------- ------------------hme0 at_elan.lc3net 00:00:a2:cb:28:5ehme0 10.1.0.79 00:e0:cf:00:0e:92hme0 st361m13 08:00:20:d8:e0:07hme0 10.1.7.103 00:90:27:b6:b5:e5hme0 10.1.0.139 00:e0:cf:00:15:bd


ARP Features

Proxy ARP– Router can answer ARP requests on network B

for a host on network A with its own MAC address since host A cannot see broadcast and B cannot send directly to A.

Gratuitous ARP– Host sends ARP for own IP address at boot.– No reply should be received.– Network misconfiguration if reply received.


DHCP

Dynamic Host Configuration Protocol provides: IP address Netmask Gateway router DNS servers


ICMP (Internet Control Message Protocol)

Network layer protocol encapsulated in IP– Communicates error messages and exceptions.– Messages handled by either IP or TCP/UDP.

IP Header (20 bytes) ICMP Message

8-bit type 8-bit code 16-bit checksum

Contents (always

depend contains

on type and code

IP header + 8 data bytes)


ICMP Message Types

Type 0: echo (ping) replyType 3: destination unreachableType 4: source quenchType 5: redirectType 8: echo (ping) requestType 9, 10: router advertisement, solicitationType 11: time (TTL) exceededType 12: parameter (header) problemType 13: timestampType 14: timestamp replyType 15, 16: information request, reply


References1. James Boney, Cisco IOS in a Nutshell, 2nd edition,

O’Reilly, 2005. 2. Cisco, Cisco Connection Documentation,

http://www.cisco.com/univercd/home/home.htm3. Cisco, Internetworking Basics,

http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/introint.htm

4. Wendell Odom, CCNA Official Exam Certification Library, 3rd edition, Cisco Press, 2007.

5. Priscilla Oppenheimer and Joseph Bardwell, Troubleshooting Campus Networks, Addison-Wesley, 2002.

6. W. Richard Stevens, TCP/IP Illustrated, Addison-Wesley, 1994.

http://www.cisco.com/univercd/home/home.htm

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