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CS 3505 the internet and the info highway IP : internet protocol brief history basic function names and addresses packet format packet routing, routing tables IPv6 : the IP of the future IP topics Internet : history ARPAnet - 1st packet switched network, 1969

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ip topics
brief history

basic function

names and addresses

packet format

packet routing, routing tables

IPv6 : the IP of the future

IP topics
internet history
Internet : history
  • ARPAnet - 1st packet switched network, 1969
  • Larry Roberts - packet switching, ARPAnet
  • BBN - built first IMPs
  • 1970s - ARPAnet grew rapidly
  • ethernet - PARC, Robert Metcalf 1970s
  • token ring, IBM - 1970s
  • TCP/IP - Vint Cerf - about 1980
ip basic function
IP : basic function
  • provides connectionless, best-effortdata delivery service to TCP/UDP/apps. Packet delivery not guaranteed.
  • Makes use of underlying networks (LANs/ WANs)
  • interface between transport layer (TCP,UDP) and the network interface (ethernet, token ring, FDDI, WAN)

--> “workhorse” of Internet; “glue” that connects many networks

ip basics
IP : basics
  • runs in routers (gateways, layer 3 switches) and hosts (end systems; computers).
  • routers are network switches which connect networks to other networks (and other routers). Mostly software. [AKA gateways]
  • transport layer (TCP,UDP) - run in hosts only, not in routers. Interface to IP.
tcp ip internet
TCP/IP internet











tcp ip protocol suite
TCP/IP protocol suite

telnet, FTP, etc.


TFTP, other apps.










ip names and addresses
IP : names and addresses
  • need unique name for every host
  • hierarchical naming structure
  • top level names assigned by InterNIC registration service
  • lower level names assigned by organization

ex: cs.nps.navy.mil

mil is the top level domain ; navy next level, nps next, etc.

ip names and addresses9
IP : names and addresses
  • some top level domains --

edu - colleges & universities

gov - US fed. gov’t agencies

com - commercial organizations in US

net - internet service organizations

org - non profit institutions

mil - U S military

countries --> jp, uk,fr, mx, de, etc.

ip names and addresses10
IP : names and addresses
  • name - series of labels, dots --




  • label can have up to 63 characters, and up to 255 characters in a name
  • worldwide naming tree -- root is top; domain is a node of the tree and its subtree
ip address
IP address
  • IP address : 32 bit number, assigned to each “host” (computer) on an IP internet.
  • switching nodes in the internet - routers - also must have IP addresses.
  • the IP address actually is assigned to interface point on the network, not the node itself .... analogous to street and house number for a home address
ip names and addresses12
IP : names and addresses
  • IP address - 2 main parts, netid and hostid
  • each part can be 1,2 or 3 bytes (class)
  • first few bits indicate which class applies
    • Class A : netid 1 byte, host id 3 bytes
    • Class B : 2 and 2
    • Class C : 1 and 3

netid hostid

ip names and addresses13
IP : names and addresses
  • dotted decimal notation -->
  • formats -> 0 (A), 10 (B), 11(C) ... in decimal, if 1st byte : 0-127 --> A, 128-191 --> B, 192-223 --> C.
  • additional classes :

D, 224-239, for multicasting

E, 240-255, reserved.

  • some address blocks reserved for networks not connected to the Internet
ip names and addresses14
IP : names and addresses
  • how many possible IP addresses, total?
  • how many class A addresses exist? B? C?
  • how many hosts are possible for each class A? B? C?
  • is this an efficient method of assigning address classes in the Internet?
ip names and addresses15
IP : names and addresses
  • suppose your organization (eg, NPS) has a class B address; you don’t have 1 big network of 64K hosts; you have numerous smaller networks, mostly LANs.
  • further -- 64K is far too many hosts for a LAN or even a LAN internet, anyway....
  • how these be separated into smaller, more manageable networks?
ip addresses subnets
IP addresses - subnets
  • the host space can be divided further into a subnet part and a host part (or system part).
  • example: NPS is 131.120.X.Y.... we can make X (3rd byte) the subnet id, and the rest (4th byte) the host id.
  • This gives room for ~255 subnets of up to 255 hosts each.... “131.120.1” is one of the CS dept subnets...
names and addresses subnets
names and addresses : subnets





NPS: 131.120

names and addresses subnets18
names and addresses : subnets
  • these different networks are connected by routers, and the NPS network is connected to the “outside” by a router.
  • how do the routers “know” which part is the subnet and hostid part?

--> subnet mask - a 32 bit string of bits; 1s correspond to the netid part (network and subnet), 0s to the system (host) part

names and addresses subnets19
names and addresses : subnets

some bit patterns are reserved for special purposes (e.g. broadcasting) , so ---

  • netid, hostid(subnetid) -- should not be all 0s or all 1s
  • netid, hostid(subnetid) -- must be at least 2 bits
ip addresses multihoming
IP addresses : multihoming
  • recall that IP address has 2 parts, the netid and hostid
  • routers, and sometimes hosts, may be connected to more than one network; which netid is the correct one?

--> both; the IP address corresponds to the network interface, not simply to the host itself. (think of a house on a corner....)

  • similarly, a host connected to 2 networks may be structured as a router
names and addresses
names and addresses
  • IP runs on top of ethernet LANs, TR LANs, etc. These rout packets according to a different address, the MAC address (not the IP address). How can IP rout packets on these networks?

--> must determine the MAC address which corresponds to a given IP address

ARPaddress resolution protocol

arp address resolution protocol
ARP :address resolution protocol
  • purpose : obtain MAC (hardware) address of a machine, given its IP address.

which MAC address has IP address



arp address resolution protocol23
ARP :address resolution protocol

IP frame “fits” into the frame of the underlying network... (“wrapper”)

IP frame



CSMA/CD frame

arp address resolution protocol24
ARP :address resolution protocol

input : IP address, i ;

output : MAC address, m;

data structure : ARP table: list of (i,m) pairs;


1. search ARP table for i ;

if found, return (m)

else broadcast ARP request (i );

2. wait for ARP reply (m);

3. when reply received, update ARP table (i,m) & return (m).


arp address resolution protocol25
ARP :address resolution protocol
  • “broadcast request” - a LAN broadcast packet, contains the ARP packet (below)

(field lengths shown in bytes)

2 2 1 1 2 6* 4 6* 4


ARP msg type


src/dest IP address

src/dest MAC address

IP/upper layer type

** ethernet length, may vary with

other protocols

MAC hardware type

arp address resolution protocol26
ARP :address resolution protocol
  • receiver part of ARP : upon receipt of an ARP request,

if the destination IP address is MA (my address), then

1. update my ARP table, as appropriate, and

2. send ARP reply.

ip packet format
IP packet format


3 4

15 16


7 8 10

source IP address

total length






fragment offset




header checksum

destination IP address

options, (if any)


ip packet explanation
IP packet - explanation
  • version -- currently 4; next - 6.
  • HLEN - header length; 20 to 60 bytes.
  • total length - packet length in bytes.
  • precedence (3 bits) - designed for priority, but no standard procedure for this; little used.
  • TOS - type of service
  • TTL - time to live (die). Standard specified seconds, but in practice - router hops.
ip packet explanation29
IP packet - explanation
  • ID - numbers each datagram sent by a host. ( fragmentation/reassembly)
  • flags - 3 bits. DF, don’t fragment; MF, more fragments. (1st bit unused=0).
  • frag offset - ( fragmentation/reassembly)
  • protocol - indicates TCP, UDP, etc.
  • header checksum - done on header only; recomputed at each hop.
ip routing
IP routing
  • routing mechanism - the mechanics of routing; simply, IP routs packets according to a routing table, in memory.
  • routing policy - how the paths in the networking are calculated- i.e., how the entries in the table are determined. Two separate procedures.
  • mechanism - differs slightly, depending on whether in a host or a router; simpler for hosts.
ip routing31
IP routing
  • basic IP routing mechanism:

given an IP DA (destination IP address),

1. search table for complete IP DA; if found, send to next hop indicated.

2. search table for network ID; if found, send to next hop indicated.

3. search for default entry; if found, send to next hop indicated.

4. discard the packet.

ip routing in a host
IP routing : in a host
  • IP (in host) receives packets to send from TCP, UDP, ICMP, IGMP.

upon receipt of a packet to send, IP will

1. check mask (determine net/host parts).

2. if destination directly connected (point-to-point link/ same subnet), then send packet to it.

3. otherwise, send packet to the default router (routing table).

ip routing in a host33
IP routing : in a host
  • fundamental difference : a host never forwards a packet; IP packets received not for this host are discarded.

note :

if sending to a host on same subnet (e.g. ethernet), the MAC address corresponds to the IP DA;

if sending to default router, the MAC/hardware DA is the router’s, while the IP DA is that of the final destination.

ip routing in a host34
IP routing : in a host

TCP, UDP, etc.

routing table





this IP DA





bit bucket



IP (host)

NW interface

ip routing in routers
IP routing : in routers

Same basic algorithm as stated, but :

  • routing tables bigger, generally ;
  • more overhead in maintaining routing tables, exchanging information with other routers;
  • more network interfaces, generally ; usually at least 2 (hosts may have only 1)
  • forward packets received onto other routers. (fundamental difference)
ip routing in routers36
IP routing : in routers

TCP, UDP, etc.

routing table





this IP DA







IP (router)

NW interfaces

ip routing tables
IP routing tables
  • series of entries which contain

destination - IP address of distant location (either network or host)

gateway(router) - IP address of router to send the packet to

flags - 5 of these which give additional info

refcnt - number of active uses

use - number of packets sent this route

interface - the outgoing interface for this route; (e.g., ethernet, a direct link, etc. )

ip routing38
IP routing
  • routing mechanism - the mechanics of routing

-- discussed previously

  • routing policy - how the paths in the network are calculated

-- there is no single required routing policy on the Internet

--DV and LS routing already discussed

-- some specific IP policies to follow

ip routing mechanism review
IP routing mechanism (review)

given an IP DA (destination IP address),

1. search table for complete IP DA; if found, send to next hop indicated.

2. search table for network ID; if found, send to next hop indicated.

3. search for default entry; if found, send to next hop indicated.

4. discard the packet.

autonomous systems
autonomous systems
  • a piece of the Internet unified by a routing policy
  • “somebody’s network”

early def: a collection of subnetworks and hosts, interconnected by routes

new def: a connected group of 1 or more IP prefixes ... which has a SINGLE and CLEARLY DEFINED routing policy

autonomous systems41
autonomous systems
  • routing within ASs is done by IGPs, or interior gateway protocols; chosen by the controlling organization
  • routing between ASs is done by EGPs, or exterior gateway protocols








ospf open shortest path first
OSPF : open shortest path first
  • link state protocol, developed by IETF; non proprietary
  • low overhead; updates report changes rather than everything
  • quick detection of topology changes, rapid updating after changes
  • traffic splitting over multiple paths
  • subnet masks supported
  • authentication supported
  • widely used, refinements will continue