Arp DNS Dhcp

Basics of IP Routing

Types of Addresses

Types of AddressesHostnames

IP Addresses

Mac Addresses

IP AddressesNumerical label assigned to each device in a computer network

Used for host or network identification and location addressing

Used as a unique identifier to identify the devices in any given network

IPV4 and IPV6 are the two versions of IP addresses

HostnameA Label assigned to the device connected in the network

URL or Uniform Resource Locator provides a reference to a resource on the internet

Mac AddressUnique identifier assigned to network interfaces for communications on the physical network segment.

Assigned by the manufacturer on the NIC

A network node may have multiple NICs and each must have one unique MAC address per NIC.

Mac addresses are of 48bits

ARP, DNS and DHCP

ARPARP associates an IP address with itsphysical address. On a typical physicalnetwork, such as a LAN, each device on alink is identified by a physical or stationaddress that is usually imprinted on theNIC.

The Reverse Address ResolutionProtocol (RARP) is an obsolete computernetworking protocol used by a clientcomputer to request its InternetProtcol (IPv4) address from a computernetwork, when all it has available isits Link Layer or hardware address, suchas a MAC address.

Encapsulation of ARP Packet

Address Translation with ARPARP Request: A broadcasts an ARP request to all stations on the network: What is the hardware address of Router137?

Host A

128.143.137.144

00:a0:24:71:e4:44

Router137

128.143.137.1

00:e0:f9:23:a8:20

ARP Request:

What is the MAC address

of 128.143.71.1?

Address Translation with ARPARP Reply: Router 137 responds with an ARP Reply which contains the hardware address

Host A

128.143.137.144

00:a0:24:71:e4:44

Router137

128.143.137.1

00:e0:f9:23:a8:20

ARP Reply:

The MAC address of 128.143.71.1

is 00:e0:f9:23:a8:20

ExampleARP Request from Host A:

Source hardware address: 00:a0:24:71:e4:44Source protocol address: 128.143.137.144Target hardware address: 00:00:00:00:00:00Target protocol address: 128.143.137.1

ARP Reply from Router137:

Source hardware address: 00:e0:f9:23:a8:20 Source protocol address: 128.143.137.1 Target hardware address: 00:a0:24:71:e4:44Target protocol address: 128.143.137.144

RARPRARP finds the logical address for a machine that only knows its physical address.

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What is DNS?DNS is a host name to IP address translation service

DNS is a distributed database implemented in a hierarchy of name servers

an application level protocol for message exchange between clients and servers

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Why DNS?It is easier to remember a host name than it is to remember an IP address.

An name has more meaning to a user than a 4 byte number.

Applications such as FTP, HTTP, email, etc., all require the user to input a destination.

The user generally enters a host name.

The application takes the host name supplied by the user and forwards it to DNS for translation to an IP address.

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DNS ServicesBesides the address translation service, DNS also provides the following services:Host aliasing: a host with a complicated name can have one or more aliases that are simpler to

remember,e.g., relay1.west-coast.media.com -> media.com. The longer name is the canonical hostname, the shorter the alias hostname.

Mail server aliasing: same as above, aliases can exist for long canonical host names.

Load Balancing: a set of servers can have one name mapped onto several machines. DNS provides the full list of names to the end users application which generally takes the first one in the list. DNS rotates the names on the list.

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Root DNS Servers

com DNS servers org DNS servers edu DNS servers

poly.edu

DNS servers

umass.edu

DNS serversyahoo.com

DNS servers

amazon.com

DNS servers

pbs.org

DNS servers

Distributed, Hierarchical Database

Client wants IP for www.amazon.com; 1st approx:

client queries a root server to find com DNS server

client queries com DNS server to get amazon.com DNS server

client queries amazon.com DNS server to get IP address for www.amazon.com

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TLD and Authoritative ServersTop-level domain (TLD) servers: responsible for com, org, net, edu, etc, and all top-level

country domains uk, fr, ca, jp.

Network Solutions maintains servers for com TLD

Educause for edu TLD

Authoritative DNS servers: organizations DNS servers, providing authoritative hostname

to IP mappings for organizations servers (e.g., Web, mail).

can be maintained by organization or service provider

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DNS: Root name serversroot name server:

contacts authoritative name server if name mapping not known

gets mapping

returns mapping to local name server

13 root

name servers

worldwideb USC-ISI Marina del Rey, CA

l ICANN Los Angeles, CA

e NASA Mt View, CA

f Internet Software C. Palo Alto, CA (and 36 other locations)

i Autonomica, Stockholm (plus

28 other locations)

k RIPE London (also 16 other locations)

m WIDE Tokyo (also Seoul,

Paris, SF)

a Verisign, Dulles, VA

c Cogent, Herndon, VA (also LA)

d U Maryland College Park, MD

g US DoD Vienna, VA

h ARL Aberdeen, MD

j Verisign, ( 21 locations)

Local Name Serverdoes not strictly belong to hierarchy

each ISP (residential ISP, company, university) has one. also called default name server

when host makes DNS query, query is sent to its local DNS server acts as proxy, forwards query into hierarchy

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DNS QueriesRecursive: The client machine sends a request to the local name server, which, if it does

not find the address in its database, sends a request to the root name server, which, in turn, will route the query to an intermediate or authoritative name server. Note that the root name server can contain some hostname to IP address mappings. The intermediate name server always knows who the authoritative name server is.

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DNS Queries (contd)Iterative:

The local server queries the root server. If address not in its database, will have the name/address of an intermediate or authoritative name server and forward that information to the local name server so that it can directly communicate with the intermediate or authoritative name server. This is to prevent the overloading of the root servers that handle millions of requests.

requesting host

cis.poly.edu

gaia.cs.umass.edu

root DNS

server

local DNS server

dns.poly.edu

1

23

4

5

6

authoritative DNS server

dns.cs.umass.edu

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TLD DNS

server

DNS name resolution example

Host at cis.poly.edu wants IP address for gaia.cs.umass.edu

iterated

query:

contacted server replies with name

of server to

contact

I dont know this name, but

ask this server

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requesting host

cis.poly.edu

gaia.cs.umass.edu

root DNS

server

local DNS server

dns.poly.edu

1

2

45

6

authoritative DNS server

dns.cs.umass.edu

7

8

TLD DNS

server

3 recursive

query:

puts burden of name resolution

on contacted name

server

heavy load?

DNS name resolution example

DNS: caching and updating recordsonce (any) name server learns mapping, it caches mapping

cache entries timeout (disappear) after some time

TLD servers typically cached in local name servers Thus root name servers not often visited

Dynamic Assignment of IP addressesDynamic assignment of IP addresses is desirable for several reasons:

IP addresses are assigned on-demand

Avoid manual IP configuration

Support mobility of laptops

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DHCPDynamic Host Configuration Protocol (DHCP)

From 1993

An extension of BOOTP, very similar to DHCP

Same port numbers as BOOTP

Extensions: Supports temporary allocation (leases) of IP addresses

DHCP client can acquire all IP configuration parameters needed to operate

DHCP is the preferred mechanism for dynamic assignment of IP addresses

DHCP can interoperate with BOOTP clients.

DHCP Interaction (simplified)Host A

00:a0:24:71:e4:44 DHCP Server

DHCP Request 00:a0:24:71:e4:44

Sent to 255.255.255.255

Host A

128.143.137.144

00:a0:24:71:e4:44 DHCP ServerDHCP Response:

IP address: 128.143.137.144

Default gateway: 128.143.137.1

Netmask: 255.255.0.0

DHCP Message TypeMessage type is sent as an option.

Value Message Type

1 DHCPDISCOVER

2 DHCPOFFER

3 DHCPREQUEST

4 DHCPDECLINE

5 DHCPACK

6 DHCPNAK

7 DHCPRELEASE

8 DHCPINFORM

Message TypesDHCPDISCOVER: Broadcast by a client to find available DHCP servers.

DHCPOFFER: Response from a server to a DHCPDISCOVER and offering IP address and other parameters.

DHCPREQUEST: Message from a client to servers that does one of the following:

Requests the parameters offered by one of the servers and declines all other offers.

Verifies a previously allocated address after a system or network change (a reboot for example).

Requests the extension of a lease on a particular address.

Contd.DHCPACK: Acknowledgement from server to client with parameters,

including IP address.

DHCPNACK: Negative acknowledgement from server to client, indicating that the client's lease has expired or that a requested IP address is incorrect.

DHCPDECLINE: Message from client to server indicating that the offered address is already in use.

DHCPRELEASE: Message from client to server canceling remainder of a lease and relinquishing network address.

DHCPINFORM: Message from a client that already has an IP address (manually configured for example), requesting further configuration parameters from the DHCP server.

DHCP Operation

DCHP DISCOVERDHCP Client


DHCPDISCOVERSent to 255.255.255.255

DHCP Server

DHCP Client


DHCP Server

DHCPOFFER

DHCPOFFERDCHP OFFER

DHCP OperationDHCP Client


DHCP Server

DHCPREQUEST

DHCPACKDCHP DISCOVER

At this time, the DHCP client can start to use the IP address

DHCP Client


DHCP Server

DHCPREQUEST

DHCPACK

Renewing a Lease

(sent when 50% of lease has expired)

If DHCP server sends DHCPNACK, then address is released.

DHCP OperationDHCP Client


DHCP Server

DHCPRELEASE DCHP RELEASE

At this time, the DHCP client has released the IP address

Circuits/ WAN Connections

Documents

Arp DNS Dhcp