Special IP Addresses

• All 0’s – this computer on bootstrap• Network.000s – id’s the network• Network.111s – broadcast• 111111 – broadcast• 127.x loopback

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Routing – again

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Address Resolution

• A data link such as Ethernet or a token ring has its own addressing scheme

• When an Ethernet frame is sent from one host to another, it is the 48-bit Ethernet address that determines the destination

• The device driver software never looks at the destination IP address in the IP datagram

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ARP

• Address resolution provides a mapping between two different forms of addresses– 32-bit IP addresses and whatever the data link uses

• ARP (address resolution protocol) is a protocol used to do address resolution in the TCP/IP protocol suite (RFC826)

• ARP provides a dynamic mapping from an IP address to the corresponding hardware address

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Basic Idea

• ARP is required on multi-access channels and relies on the ability to broadcast

• The protocol is simple:– broadcast a packet containing the IP address of the

destination machine

– the machine with that address, or possibly a server, sends a reply containing the hardware address

– upon receipt the hardware address is used to send the original packet

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ARP Cache

• Essential to the efficient operation of ARP is the maintenance of a cache on each host

• The cache maintains the recent IP to physical address mappings

• Each entry is aged (usually the lifetime is 20 minutes) forcing periodic updates of the cache

• ARP replies are often broadcast so that all hosts can update their caches

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arp Commnd

• The arp(8) command on a Unix system can be used to see the contents of the ARP cachekiev> arp -aNet to Media TableDevice IP Address Mask Flags Phys Addr------ -------------------- --------------- ----- ---------------hme0 redshirt 255.255.255.255 00:60:08:8b:e9:aahme0 tiger 255.255.255.255 08:00:20:85:f6:8dhme0 thunderbolt 255.255.255.255 08:00:20:9a:af:60hme0 starfury 255.255.255.255 08:00:20:9a:af:79hme0 cs3-router 255.255.255.255 00:10:11:09:f0:28hme0 epsilon3-38 255.255.255.255 08:00:20:86:71:c0hme0 mordor-38 255.255.255.255 08:00:20:96:01:adhme0 itlabman 255.255.255.255 00:00:c0:65:d7:b9hme0 joanne 255.255.255.255 00:05:02:59:51:52hme0 laurie 255.255.255.255 00:05:02:79:c4:20hme0 kiev 255.255.255.255 SP 08:00:20:9e:f2:99

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ARP Packet Format168

Sender’s Protocol Address

Destination IP Address

31

Hardware Type

Hardware Size Protocol Size Operation

Protocol Type

Sender’s Hardware Address (for Ethernet 6 bytes)

(for IP 4 bytes) Target Hardware Address

Target Protocol Address

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Hardware type (HTYPE)This field specifies the Link Layer protocol type. Example: Ethernet is 1.Protocol type (PTYPE)This field specifies the upper layer protocol for which the ARP request is intended. For IPv4, this has the value 0x0800. The permitted PTYPE values share a numbering space with those for Ethertype.[3][4][5]Hardware length (HLEN)Length (in octets) of a hardware address. Ethernet addresses size is 6.of the intended receiver.

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Protocol length (PLEN)Length (in octets) of addresses used in the upper layer protocol. (The upper layer protocol specified in PTYPE.) IPv4 address size is 4.Operation Specifies the operation that the sender is performing: 1 for request, 2 for reply.Sender hardware address (SHA)Hardware (MAC) address of the sender.Sender protocol address (SPA)Upper layer protocol address of the sender.Target hardware address (THA)Hardware address of the intended receiver. This field is ignored in requests.Target protocol address (TPA)Upper layer protocol address

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Proxy ARP

• Proxy ARP lets a router answer ARP requests on one of its networks for a host on another of its networks

• This fools the sender of the ARP request into thinking that the router is the destination

• The router is acting as a proxy agent for the destination, relaying packets to it from other hosts

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Proxy ARP

• Proxy ARP is also known as promiscuous ARP or the ARP hack

• The names come from the other use of proxy ARP: to hide two physical networks from each other, with a router between the two

• This has been used to separate hosts running two different versions of TCP/IP

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Gratuitous ARP

• Gratuitous ARP occurs when a host sends an ARP request looking for its own IP address

• This can happen at bootstrap time• Gratuitous ARP provides two features

– it lets a host determine if another host is already configured with the same IP address

– if the host sending the gratuitous ARP has just changed its hardware address, the packet causes other hosts on the net to update their ARP cache entries

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Issues

• Many people ARP to be a dangerous protocol– a bogus host can issue a gratuitous ARP and change

cache entries

– a bogus host can send replies giving its own hardware address (instead of the target)

• Broadcasting can be expensive– excessive use of bandwidth

– CPU costs

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Reverse Address Resolution Protocol

• When a system boots, it typically gets its IP address from a file

• How does a system, without a disk, get its IP address?

• Since each system has a unique hardware address, that hardware address can be used to lookup the corresponding IP address

• RARP (RFC903) does exactly that

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RARP Packet Format

• The format is exactly the same as ARP except some of the numbers change

• The RARP request is broadcast and the reply is sent to the requester

• Unlike ARP, designated RARP server(s) that handles RARP requests

DHCP (Dynamic Host Control)

• Proceeded by BOOTP• BOOTP Required that a server know the

machine’s IP before it booted• Broadcast is sent from a host, DHCP server

responds• IP addresses are given out on a lease, which

expires. Once expired, lease terminates or can be renogiated

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DHCP Request

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DHCP Step 3

• Send an ACK or a NACK

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Problems

• What are some of the problems of IPv4?• How can we fix them?

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Networking Technologies

• SONET – designed to carry digital voice telephone Synchronous Optical Networking

• Optical Carrier (OC) • DSL (Digital Subscriber Line) and Cable Modems• Virtual Network

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