Transcript IP: Adressing, ARP, and Routing

IP: Addressing, ARP, Routing
Network Protocols and Standards
Autumn 2004-2005
Oct 21, 2004
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IPv4
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IP Datagram Format
IPv4 Addressing
ARP and RARP
IP Routing Basics
Subnetting and Supernetting
ICMP
Network Address Translation (NAT)
Dynamic Addressing
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Internet Addresses (IP Addresses)
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Defined when IP was standardized in 1981
IP addresses are 32-bit long and consist of:
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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
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Internet Address Classes
0
8
Class A
0
Class B
10
Class C
110
Class D
1110
Class E
11110
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16
NETWORK
24
32 bits
HOST (24 BITS)
NETWORK
HOST (16 BITS)
NETWORK
HOST (8 BITS)
IP MULTICAST ADDRESSES (28 BITS)
RESERVED FOR EXPERIMENTS
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Internet Address Classes
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Class A
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Class B
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126 networks (0 and 127 reserved)
Assigned to very large size networks where 65K < number
of hosts < 16M
16384 networks
Assigned to Intermediate size networks where 256 <
number of hosts < 65K
Class C
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2097152 networks
Assigned to smaller networks where #hosts < 256
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Dotted Decimal Notation
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Internet addresses are represented in
text by the dotted decimal notation
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each byte is written in decimal values
(from 0 to 255)
example:
10000000 00001010 00000010 00011110
is written as 128. 10. 2. 30
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Internet Address Classes
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Class
Lowest Network
Identifier Address
Highest Network
Identifier Address
A
1.0.0.0
126.0.0.0
B
128.0.0.0
191.255.0.0
C
192.0.0.0
223.255.255.0
D
224.0.0.0
239.255.255.255
E
240.0.0.0
247.255.255.255
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Uniqueness of IP Addresses
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Network numbers are assigned by a central authority
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Network numbers are unique worldwide
Host numbers are assigned by network managers
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The Internet Network Information Center (InterNIC)
Another authority, the IANA – Internet Assigned Numbers
Authority sets policy
They must be unique within a given network
Thus, IP addresses are unique worldwide.
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Special Purpose IP Addresses
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0.0.0.0
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255.255.255.255
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Means the host on this network
Host part all ones
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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
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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)
Broadcast address on the network specified in the network identifier;
routers typically do not forward these datagrams
Host part all zeros
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Broadcast address on the network specified in the network identifier (it was
an implementation error in some networks)
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Special Purpose IP Addresses
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127.x.x.x
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224.0.0.1
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Means loopback (datagrams are looped back in
software; they are not sent on any physical
interface)
Multicast address for “All systems on this
subnetwork”
224.0.0.2
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Multicast address for “All routers on this
subnetwork”
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Internet Addresses
ALL 0’s
ALL 0’s
This Host
HOST
Limited broadcast (local net)
ALL 1’s
NET
127
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ALL 1’s
ANYTHING
Host on this network
Directed broadcast (for NET)
Loopback
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Translating Between IP and MAC
Addresses (ARP and RARP)
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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
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Address Resolution Protocol
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Used to find the physical address of a target device on the local
physical network, given only the target’s IP address
Mechanism:
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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)
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More on ARP
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ARP refinements
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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
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ARP Message Format
0
8
16
24
HARDWARE TYPE
HLEN
31
PROTOCOL TYPE
PLEN
OPERATION
SENDER HA (octets 0-3)
SENDER HA (octetS 4-5)
SENDER IP (octetS 0-1)
SENDER IP (octetS 2-3)
TARGET HA (octets 0-1)
TARGET HA (octets 2-5)
TARGET IP (octets 0-3)
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ARP Message
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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. 0800 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
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Reverse ARP
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Usually, a machine’s IP address is kept on its
secondary storage (OS finds it at start up)
Issue : Diskless Workstations!
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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
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RARP
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Mechanism
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Mechanism allows a host to ask about an arbitrary
target
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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
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
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IP Routing
Network Protocols and Standards
Autumn 2002-2003
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Routing IP Datagram
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Direct Delivery (i.e., not involving routers):
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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
Router
Example:
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A sends IP Datagram to B
A
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C
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Routing IP Datagram
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Indirect delivery (i.e. through intermediate
routers)
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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).
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Direct and Indirect Routing
B wants to send packets to A and C!
Host A
204.240.18.10
204.240.18.1
Direct Routing:
Packets sent directly using
MAC address of A
Host B
204.240.18.20
Router
Internet
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 C
36.14.0.200
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IP Routing Decisions
10.0.0.5
Network
10.0.0.0
20.0.0.6
20.0.0.5
R1
40.0.0.7
30.0.0.6
R3
Network
20.0.0.0
Network
30.0.0.0
R2
30.0.0.7
Network
40.0.0.0
Routing Table of R2
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To Reach Hosts on Network
Next Hop Address
20.0.0.0
Direct Delivery
30.0.0.0
Direct Delivery
10.0.0.0
20.0.0.5
40.0.0.0
30.0.0.7
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IP Routing Algorithm
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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
 Use next hop in that entry
Else If default route for next hop exists
 Use default route for next hop
Else
 Declare error
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Routing Within Same Network
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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?
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Proxy ARP
To Internet
Main Router
Main Network
A
B
C
Router R
D
E
Hidden Network
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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
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Proxy ARP
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Advantage of Proxy ARP Router
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Disadvantages:
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Can be added without disturbing the routing table in other
hosts or routers on that network
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:
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Several IP addresses map to the same physical address.
How to distinguish between a legitimate Proxy ARP router
and spoofing?
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