Network Layer Routing (Forwarding) Logic
Routers and end-user computers (called hosts in a TCP/IP network) work together to perform
IP routing.The host operating system (OS) has TCP/IP software, including the software that
implements the network layer. Hosts use that software to choose where to send IP packets,
oftentimes to a nearby router.
The IP packet, created by PC1, goes from the top of the figure all the way to PC2 at the bottom
of the figure
IPv4 Addressing
If a device wants to communicate using TCP/IP, it needs an IP address. When the device has an
IP address and the appropriate software and hardware, it can send and receive IP packets. Any
device that has at least one interface with an IP address can send and receive IP packets and is
called an IP host.
The original specifications for TCP/IP grouped IP addresses into sets of consecutive addresses
called IP networks. The addresses in a single IP network have the same numeric value in the first
part of all addresses in the network.
Some Rules
IP addresses consist of a 32-bit number, usually written in dotted-decimal notation (DDN). The
“decimal” part of the term comes from the fact that each byte (8 bits) of the 32-bit IP address
is shown as its decimal equivalent. The four resulting decimal numbers are written in sequence,
with “dots,” or decimal points, separating the numbers—hence the name dotted-decimal. For
example, 168.1.1.1 is an IP address written in dotted-decimal form; the actual binary version is
10101000 00000001 00000001 00000001.
Each DDN has four decimal octets, separated by periods. The term octet is just a vendor-neutral
term for byte. Because each octet represents an 8-bit binary number, the range of decimal numbers
in each octet is between 0 and 255, inclusive. For example, the IP address of 168.1.1.1 has
a first octet of 168, the second octet of 1, and so on.
The IPv4 address space includes all possible combinations of numbers for a 32-bit IPv4 address.
Literally 232 different values exist with a 32-bit number, for more than 4 billion different numbers.
With DDN values, these numbers include all combinations of the values 0 through 255 in
all four octets: 0.0.0.0, 0.0.0.1, 0.0.0.2, and all the way up to 255.255.255.255.
Class A, B, and C IP Networks
Ip standards first subdivide the entire address space into classes, as identified by the value of the
first octet. Class A gets roughly half of the IPv4 address space, with all DDN numbers that begin
with 1–126.Class B gets one-fourth of the address space, with all DDN numbers that begin with 128–191 inclusive, while Class C gets one-eighth of the address space,
with all numbers that begin with 192–223.
IP Subnetting
Subnetting defines methods of further subdividing the IPv4 address space into groups that are
smaller than a single IP network. IP subnetting defines a flexible way for anyone to take a single
Class A, B, or C IP network and further subdivide it into even smaller groups of consecutive
IP addresses. In fact, the name subnet is just shorthand for subdivided network. Then, in each
location where you used to use an entire Class A, B, or C network, you can use a smaller subnet,
wasting fewer IP addresses.
IPv4 Routing
The routing example uses pic4. In this example, all routers happen to use the Open
Shortest Path First (OSPF) routing protocol, and all routers know routes for all subnets. In particular,
PC2, at the bottom, sits in subnet 150.150.4.0, which consists of all addresses that begin
with 150.150.4.
The routing (forwarding) process depends heavily on having an accurate and up-to-date IP routing
table on each router.
Routing Protocol
To dynamically learn and fill the routing table with a route to each subnet in the
internetwork.
If more than one route to a subnet is available, to place the best route in the routing table.
To notice when routes in the table are no longer valid, and to remove them from the routing
table.
If a route is removed from the routing table and another route through another neighboring
router is available, to add the route to the routing table. (Many people view this goal and the
preceding one as a single goal).
To work quickly when adding new routes or replacing lost routes. (The time between losing
the route and finding a working replacement route is called convergence time.)
To prevent routing loops.
Routers and end-user computers (called hosts in a TCP/IP network) work together to perform
IP routing.The host operating system (OS) has TCP/IP software, including the software that
implements the network layer. Hosts use that software to choose where to send IP packets,
oftentimes to a nearby router.
The IP packet, created by PC1, goes from the top of the figure all the way to PC2 at the bottom
of the figure
IPv4 Addressing
If a device wants to communicate using TCP/IP, it needs an IP address. When the device has an
IP address and the appropriate software and hardware, it can send and receive IP packets. Any
device that has at least one interface with an IP address can send and receive IP packets and is
called an IP host.
The original specifications for TCP/IP grouped IP addresses into sets of consecutive addresses
called IP networks. The addresses in a single IP network have the same numeric value in the first
part of all addresses in the network.
Some Rules
IP addresses consist of a 32-bit number, usually written in dotted-decimal notation (DDN). The
“decimal” part of the term comes from the fact that each byte (8 bits) of the 32-bit IP address
is shown as its decimal equivalent. The four resulting decimal numbers are written in sequence,
with “dots,” or decimal points, separating the numbers—hence the name dotted-decimal. For
example, 168.1.1.1 is an IP address written in dotted-decimal form; the actual binary version is
10101000 00000001 00000001 00000001.
Each DDN has four decimal octets, separated by periods. The term octet is just a vendor-neutral
term for byte. Because each octet represents an 8-bit binary number, the range of decimal numbers
in each octet is between 0 and 255, inclusive. For example, the IP address of 168.1.1.1 has
a first octet of 168, the second octet of 1, and so on.
The IPv4 address space includes all possible combinations of numbers for a 32-bit IPv4 address.
Literally 232 different values exist with a 32-bit number, for more than 4 billion different numbers.
With DDN values, these numbers include all combinations of the values 0 through 255 in
all four octets: 0.0.0.0, 0.0.0.1, 0.0.0.2, and all the way up to 255.255.255.255.
Class A, B, and C IP Networks
Ip standards first subdivide the entire address space into classes, as identified by the value of the
first octet. Class A gets roughly half of the IPv4 address space, with all DDN numbers that begin
with 1–126.Class B gets one-fourth of the address space, with all DDN numbers that begin with 128–191 inclusive, while Class C gets one-eighth of the address space,
with all numbers that begin with 192–223.
IP Subnetting
Subnetting defines methods of further subdividing the IPv4 address space into groups that are
smaller than a single IP network. IP subnetting defines a flexible way for anyone to take a single
Class A, B, or C IP network and further subdivide it into even smaller groups of consecutive
IP addresses. In fact, the name subnet is just shorthand for subdivided network. Then, in each
location where you used to use an entire Class A, B, or C network, you can use a smaller subnet,
wasting fewer IP addresses.
IPv4 Routing
The routing example uses pic4. In this example, all routers happen to use the Open
Shortest Path First (OSPF) routing protocol, and all routers know routes for all subnets. In particular,
PC2, at the bottom, sits in subnet 150.150.4.0, which consists of all addresses that begin
with 150.150.4.
The routing (forwarding) process depends heavily on having an accurate and up-to-date IP routing
table on each router.
Routing Protocol
To dynamically learn and fill the routing table with a route to each subnet in the
internetwork.
If more than one route to a subnet is available, to place the best route in the routing table.
To notice when routes in the table are no longer valid, and to remove them from the routing
table.
If a route is removed from the routing table and another route through another neighboring
router is available, to add the route to the routing table. (Many people view this goal and the
preceding one as a single goal).
To work quickly when adding new routes or replacing lost routes. (The time between losing
the route and finding a working replacement route is called convergence time.)
To prevent routing loops.
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