1. IP: Addressing, ARP, Routing
Autumn
Oct 21, 2004
IP: Addressing, ARP, Routing
Network Protocols and Standards
Autumn
Oct 21, 2004
CS573: Network Protocols and Standards
CS573: Network Protocols and Standards

2. CS573: Network Protocols and Standards
IPv4
IP Datagram Format
IPv4 Addressing
ARP and RARP
IP Routing Basics
Subnetting and Supernetting
ICMP
Network Address Translation (NAT)
Dynamic Addressing
Oct 21, 2004
CS573: Network Protocols and Standards

3. Internet Addresses (IP Addresses)
Defined when IP was standardized in 1981
IP addresses are 32-bit long and consist of:
a network address part – network identifier
a host address part – host number within that network
IP addresses are grouped into classes (A,B,C) depending on the size of the network identifier and the host part of the address
A fourth class (Class D) was defined later (1988) for Multicast addresses
Oct 21, 2004
CS573: Network Protocols and Standards

4. Internet Address Classes
bits
NETWORK
HOST (24 BITS)
Class A
Class B
Class C
Class D
Class E 10
NETWORK
HOST (16 BITS)
110
NETWORK
HOST (8 BITS)
1110
IP MULTICAST ADDRESSES (28 BITS)
11110
RESERVED FOR EXPERIMENTS
Oct 21, 2004
CS573: Network Protocols and Standards

5. Internet Address Classes
Class A
126 networks (0 and 127 reserved)
Assigned to very large size networks where 65K < number of hosts < 16M
Class B
16384 networks
Assigned to Intermediate size networks where 256 < number of hosts < 65K
Class C
networks
Assigned to smaller networks where #hosts < 256
Oct 21, 2004
CS573: Network Protocols and Standards

6. Dotted Decimal Notation
Internet addresses are represented in text by the dotted decimal notation
each byte is written in decimal values (from 0 to 255)
example:
is written as
Oct 21, 2004
CS573: Network Protocols and Standards

7. Internet Address Classes
Lowest Network Identifier Address
Highest Network Identifier Address A B C D E
Oct 21, 2004
CS573: Network Protocols and Standards

8. Uniqueness of IP Addresses
Network numbers are assigned by a central authority
The Internet Network Information Center (InterNIC)
Another authority, the IANA – Internet Assigned Numbers Authority sets policy
Network numbers are unique worldwide
Host numbers are assigned by network managers
They must be unique within a given network
Thus, IP addresses are unique worldwide.
Oct 21, 2004
CS573: Network Protocols and Standards

9. Special Purpose IP Addresses
Means this host, used by machines as source address when they boot up (if they don’t know their IP address, and need to get it from a boot server)
Means limited broadcast. Used as a destination address to send packets to all hosts on the local network where the source is. Packets sent to this address are never relayed
Network part all zeros
Means the host on this network
Host part all ones
Broadcast address on the network specified in the network identifier; routers typically do not forward these datagrams
Host part all zeros
Broadcast address on the network specified in the network identifier (it was an implementation error in some networks)
Oct 21, 2004
CS573: Network Protocols and Standards

10. Special Purpose IP Addresses
127.x.x.x
Means loopback (datagrams are looped back in software; they are not sent on any physical interface)
Multicast address for “All systems on this subnetwork”
Multicast address for “All routers on this subnetwork”
Oct 21, 2004
CS573: Network Protocols and Standards

11. CS573: Network Protocols and Standards
Internet Addresses
ALL 0’s
This Host
Host on this network
Limited broadcast (local net)
Directed broadcast (for NET)
Loopback
ALL 0’s
HOST
ALL 1’s
NET
ALL 1’s
127
ANYTHING
Oct 21, 2004
CS573: Network Protocols and Standards

12. Translating Between IP and MAC Addresses (ARP and RARP)
Each interface has an IP address at Layer 3, and a MAC address at Layer 2
Assume that host A wants to send a packet to host B (A and B on the same network)
Host A knows the IP address of host B; however, in order to transmit the packet, host A must somehow know or find out what the MAC (layer 2) address of host B is!
Solution: the Address Resolution Protocol (ARP), RFC826
Oct 21, 2004
CS573: Network Protocols and Standards

13. Address Resolution Protocol
Used to find the physical address of a target device on the local physical network, given only the target’s IP address
Mechanism:
The source broadcasts a special packet asking the device with target IP address to respond with a message carrying the (IP address, physical address) mapping
All devices on the local physical network receive the broadcast, but only the target recognizes its IP address and responds to the request
When the source receives the reply, it sends the packet to the target using the target’s physical address and places the mapping in its cache (a cache is used to prevent repeated broadcasts for the same destination)
Oct 21, 2004
CS573: Network Protocols and Standards

14. CS573: Network Protocols and Standards
More on ARP
ARP refinements
Source includes its <IP address, physical address> mapping in the ARP request anticipating the target’s need for it in the near future. This avoids extra network traffic
When all machines receive the ARP request broadcast, they can store the address mapping in their cache. Do they?
ARP is used when an IP to physical address mapping changes to notify hosts on the network of the change
ARP messages are encapsulated in MAC frames. A special value in the type field of the frame is used to indicate that it is carrying an ARP message (0806 hex is used for ARP)
Entries in the local ARP cache for each host time out after a certain period
Oct 21, 2004
CS573: Network Protocols and Standards

15. CS573: Network Protocols and Standards
ARP Message Format
PROTOCOL TYPE
TARGET IP (octets 0-3) 16 8 24
HLEN
HARDWARE TYPE
SENDER HA (octets 0-3)
TARGET HA (octets 2-5) 31
PLEN
OPERATION
SENDER IP (octetS 0-1)
SENDER HA (octetS 4-5)
TARGET HA (octets 0-1)
SENDER IP (octetS 2-3)
Oct 21, 2004
CS573: Network Protocols and Standards

16. CS573: Network Protocols and Standards
ARP Message
HARDWARE TYPE: specifies type of hardware interface for which the request is made (e.g., 1 for Ethernet)
PROTOCOL TYPE: specifies high level protocol address supplied in message (e.g hex for IP)
HLEN and PLEN: specify length of fields for hardware address and protocol address respectively
OPERATION: specifies if this is an ARP request or reply message (1 for ARP request, 2 for ARP response, 3 for RARP request and 4 for RARP response)
HA and IP: hardware and IP addresses respectively
Oct 21, 2004
CS573: Network Protocols and Standards

17. CS573: Network Protocols and Standards
Reverse ARP
Usually, a machine’s IP address is kept on its secondary storage (OS finds it at start up)
Issue : Diskless Workstations!
files are stored on a remote server
need IP address to use TCP/IP to obtain initial boot image
Solution : Use physical address to identify machine
Given a physical network address, find the corresponding Internet address
Reverse Address Resolution Protocol (RARP), RFC903
Oct 21, 2004
CS573: Network Protocols and Standards

18. CS573: Network Protocols and Standards
RARP
Mechanism
Sender broadcasts a RARP request, supplying its physical network address in the Target HA field
Only machines authorized to supply the RARP service (RARP servers) process the request and send a reply filling in the target internet address
Mechanism allows a host to ask about an arbitrary target
thus sender HA is separate from target HA address
RARP server replies to sender’s HA
Ethernet frame Protocol Type for RARP is 8035 hex
Oct 21, 2004
CS573: Network Protocols and Standards

19. Autumn 2004-2005 Network Protocols and Standards Autumn 2002-2003
Oct 21, 2004
IP Routing
Network Protocols and Standards
Autumn
Oct 21, 2004
CS573: Network Protocols and Standards
CS573: Network Protocols and Standards

20. CS573: Network Protocols and Standards
Routing IP Datagram
Direct Delivery (i.e., not involving routers):
Transmission of an IP datagram between two machines on a single physical network does not involve routers
The sender encapsulates the datagram in a physical frame, binds the destination IP address to a physical hardware address (using ARP), and sends the resulting frame directly to the destination
The two machines are known to be on the same network because they have the same network identifier
Example:
A sends IP Datagram to B
Router A B C
Oct 21, 2004
CS573: Network Protocols and Standards

21. CS573: Network Protocols and Standards
Routing IP Datagram
Indirect delivery (i.e. through intermediate routers)
Host performs routing decisions based on routing table indicating “next hop”
“Next hop” refers to next router IP address on this network, via which the destination is reached
Routing decisions are made based on network prefixes (not full IP address)
The sender encapsulates the datagram in a frame with the router’s physical destination address (which is found by means of ARP).
Oct 21, 2004
CS573: Network Protocols and Standards

22. Direct and Indirect Routing
B wants to send packets to A and C!
Host A
Internet
Router
Direct Routing:
Packets sent directly using
MAC address of A
Indirect Routing:
Packets sent to the MAC address
of the router. At the IP level, B is
The source and C is the destination
Host B
Host C
Oct 21, 2004
CS573: Network Protocols and Standards

23. IP Routing Decisions R3 R1 R2 Routing Table of R2
Network R3
Network
Network
Network R1 R2
Routing Table of R2
To Reach Hosts on Network
Next Hop Address
Direct Delivery
Oct 21, 2004
CS573: Network Protocols and Standards

24. CS573: Network Protocols and Standards
IP Routing Algorithm
Router receives an IP datagram with network portion N and destination D
If N is directly connected
Transmit on that network
Else If host specific entry for D exists
Use next hop in that entry
Else If route entry for N exists
Else If default route for next hop exists
Use default route for next hop
Else
Declare error
Oct 21, 2004
CS573: Network Protocols and Standards

25. Routing Within Same Network
Consider a small company with a single LAN to which a class C network address has been assigned
The company is interested in adding another small physical network (connected to old network through a router) with a few hosts
Question: Could this company assign these hosts IP addresses from the same C class network? i.e., could the two LANs share the same class C network address?
Oct 21, 2004
CS573: Network Protocols and Standards

26. CS573: Network Protocols and Standards
Proxy ARP
Used to allow two physical networks to share the same IP network prefix
Router R’s table is configured manually to route between these two networks
Router R answers ARP requests on each network for hosts on the other network, giving its own hardware address as the target address
Main Router
To Internet
Main Network A B C
Router R D E
Hidden Network
Oct 21, 2004
CS573: Network Protocols and Standards

27. CS573: Network Protocols and Standards
Proxy ARP
Advantage of Proxy ARP Router
Can be added without disturbing the routing table in other hosts or routers on that network
Disadvantages:
Does not generalize to complex network topologies (does not scale)
Does not support a reasonable form of routing. (relies on network managers to maintain tables of machines and addresses manually)
Issues:
Several IP addresses map to the same physical address. How to distinguish between a legitimate Proxy ARP router and spoofing?
Oct 21, 2004
CS573: Network Protocols and Standards