Download Logical Addressing

Internet Protocol
How does information get sent from
one device to another across a WAN?
The network layer
• The purpose of the physical layer is to provide a
physical mechanism for transmitting data as
signals.
• The purpose of the data link layer is to ensure the
integrity of the data sent over a given physical
link.
• The purpose of the network layer is to provide a
mechanism for data to be sent from one device to
another over a route that might span many
different physical links.
• Routing is the process by which data is directed
across multiple links from one host to another.
Logical Addressing
• The data link layer uses physical addressing,
such as MAC or NIC addresses, where the
address is tied to a specific physical device.
• Network layer protocols use logical addressing,
where a given device can have multiple possible
addresses.
IP Addresses
• Each connection to the Internet is given an IP
address of some for or another.
• No two connections on the Internet can have the
same IP address at the same time.
• However, these are still logical addresses because
a given connection’s IP address can change, or it
can be reassigned to a new device if the first one is
disconnected from the Internet.
• If a given device has multiple connections to the
internet (such as a router), each connection needs
its own IP address.
• Old school addressing:
– All current IP addresses are 4 bytes.
– Once upon a time, IP address were arranged somewhat
hierarchically:
– The first byte would indicate the class of the site, usually
reflective of its size.
– The second byte would further uniquely identify the site.
– The third byte would be the subnet within the site.
– The fourth byte would specify the actual machine.
– This was wasteful. Small organizations like Carthage
did not need 65,546 IP addresses. Lots of potential
addresses were being wasted.
– This was fine when the number of machines connected
to the internet could be measured in the hundredthousands.
– Now that it’s creeping up on the billions, not so fine.
Modern addressing
• Modern addressing largely does away with
the hierarchy.
• Instead, organizations are assigned address
blocks.
• These are groups of contiguous addresses
• A block must contain 2n addresses, where n ranges
from 0 to 15.
• The first address in a block must be evenly divisible
by the number of addresses.
• The first address in block is the network address,
which is the address that identifies the entire
organization to the Internet, particularly routers.
• Within an address block, the organization is free to
divide and allocate those addresses however it sees
fit.
• The network address must point to the network’s
router, which is responsible for forwarding packets
out to the appropriate machines on its network.
• Some organizations may choose to subnet their
network. They divide their address block into
smaller blocks. However, the rules for subnetting
are the same as the ones for assigning address
blocks.
Network Address Translation
• For small organizations such as your house or Carthage College,
the ideal scenario is to have each one use as few IP addresses as
possible.
• The NAT protocol achieves this by, in the limiting case, allowing
an entire organization to be represented to the Internet by one IP
address.
• Certain IP address blocks have been set aside as private networks,
where anyone can use them.
• A NAT system has one device connected to the internet, such as a
router or wireless router, which has a single IP address.
• Devices behind the NAT router are assigned private network IP
addresses by the NAT router. All traffic to these devices goes
through the NAT router.
IPv6 addressing
• It is almost a certainty that we will run out
of IP addresses some day. IPv6 tries to
address this by expanding the address space
available.
• IP addresses were 32 bits. IPv6 addresses
are 128 bits.
IP
• IP is a packet-switched protocol.
• IP is a connectionless protocol - each packet
is treated separately.
• No flow or error control - best-effort
delivery.
• Being paired with TCP makes it reliable.
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