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Copyright 2002, Marchany
TCP/IP Review
Randy Marchany
VA Tech Computing Center
Spring, 2001
Copyright 2002, Marchany
Internet Addresses
Hosts are identified by names, addresses and routes
Internet Address: 32bit integer Address is a pair: (netid, hostid)
– 5 classes of addresses (A,B,C,D,E)
Routers use the netid to decide where to send the packet
Copyright 2002, Marchany
Internet Addresses
Multi-homed hosts: hosts that have >1 net connection need an IP address for each I/F.
Broadcast Address: refers to ALL hosts on a net.
Subnets: used to conserve IP addresses
Loopback Address: 127.0.0.0
Copyright 2002, Marchany
Network Byte Order A Standard representation of data LITTLE ENDIAN
– Lowest memory address contains the low order byte of the integer 0----------15
BIG ENDIAN– Lowest memory address contains the high order
byte of the integer 15-----------0 Each host converts from internal format to
NBO before Xmit. User data is exempt. NBO is BIG ENDIAN.
Copyright 2002, Marchany
Internet Architecture
A collection of networks 2 networks can only be connected by a
computer that is connected to both of them. This machine is a router.
Routers use the destination network address not the destination host address when routing a packet.
The amount of info a router needs to store is proportional to the # of nets not hosts.
Copyright 2002, Marchany
Address Resolution Protocol
ARP maps internet addresses to physical addresses. (RFC 826, 814, 1029, 1166)
2 types of Physical Addresses– ETHERNET – 48 bit address
• You change the I/F, you change the Ethernet Address.
– PRONET – Token Ring• Allows the user to choos a HW address when
installing the I/F board.
Copyright 2002, Marchany
Address Resolution Protocol ARP is a low level protocol that maps
addresses dynamically. ARP allows a host to find the physical
address of the target on the SAME physical net given the IP address only.
ARP Cache – Contains recent ARP bindings to improve
efficiency.– Sender includes its own (Pa, Ia) pair when it sends
the ARP packet. Target can save time with the info ahead of time.
Copyright 2002, Marchany
Address Resolution Protocol
A wants B’s physical address (Pb) A broadcasts an ARP packet that ask
host Ib to respond w/its Pb. All hosts on the subnet receive the
message but only B responds. (Unicast) A gets Pb and uses this address to
send packets to B directly.
Copyright 2002, Marchany
ARP Implementation
REQUEST– Given Ib, consult cache for Pb. If so,
extract Pb and continue with transmission.– OW, broadcast ARP request packet.
Consider delays in packet transmission. REPLY
– Receive the ARP packet, extract (Ia, Pa), look in cache and update it.
– Are we the target? If so, send (Ib,Pb) back to sender using (Ia, Pa) pair.
