Sem1 - Module 10
Routing Fundamentals and Subnets
Review
Routable and routed protocols:
A protocol is a set of rules that determines how computers communicate
with each other across networks.
A protocol describes the following:
– The format that a message must conform to
– The way in which computers must exchange a message within
the context of a particular activity
A routed protocol allows the router to forward data between nodes on
different networks.
Routed Protocols:
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IP (Internet Protocol)
IPX/SPX
AppleTalk
DECnet
AppleTalk
Banyan VINES
Xerox Network Systems (XNS)
IP as a routed protocol:
encapsulation
Routable and routed protocols:
As a packet travels through an internetwork to
its final destination, the Layer 2 frame headers
and trailers are removed and replaced at every
Layer 3 device.
This is because Layer 2 data units, frames, are
for local addressing.
Layer 3 data units, packets, are for end-to-end
addressing.
encapsulation
de-encapsulation
Routable and routed protocols:
As a frame is received at a router interface, the destination MAC
address is extracted.
The address is checked to see if the frame is directly addressed to the
router interface, or if it is a broadcast.
In either of these two cases, the frame is accepted. Otherwise, the
frame is discarded since it is destined for another device on the
collision domain.
The accepted frame has the Cyclic Redundancy Check (CRC)
information extracted from the frame trailer, and calculated to verify that
the frame data is without error.
If the check fails, the frame is discarded. If the check is valid, the frame
header and trailer are removed and the packet is passed up to Layer 3.
The packet is then checked to see if it is actually destined for the router,
or if it is to be routed to another device in the internetwork.
If the destination IP address matches one of the router ports, the Layer
3 header is removed and the data is passed up to the Layer 4.
Routable and routed protocols:
Some protocols, such as IPX, require only a network number because
these protocols use the host's MAC address for the host number.
Other protocols, such as IP, require a complete address consisting of a
network portion and a host portion.
These protocols also require a network mask in order to differentiate
the two numbers.
The network address is obtained by ANDing the address with the
network mask.
Consider following address & SNM:
192.168.25.79/27:
192.168.25.79 255.255.255.224
IP:
11000000. 10101000.00011001.01001111
SNM:
11111111.11111111.11111111.11100000
SubNet Addr:
11000000. 10101000.00011001.01000000
192.168.25.64 (Subnet for the IP 192.168.25.79/27)
Anatomy of an IP packet:
The IP header consists of the following:
Version – Indicates the version of IP currently used; four bits. If the
version field is different than the IP version of the receiving device, that
device will reject the packets.
IP header length (HLEN) – Indicates the datagram header length in
32-bit words. This is the total length of all header information,
accounting for the two variable-length header fields.
Total length – Specifies the length of the entire packet in bytes,
including data and header, 16 bits. To get the length of the data
payload subtract the HLEN from the total length.
Anatomy of an IP packet:
The IP header consists of the following:
Flags – A three-bit field in which the two low-order bits control
fragmentation. One bit specifies whether the packet can be
fragmented, and the other specifies whether the packet is the last
fragment in a series of fragmented packets.
Time-to-live (TTL) – A field that specifies the number of hops a packet
may travel. This number is decreased by one as the packet travels
through a router. When the counter reaches zero the packet is
discarded. This prevents packets from looping endlessly.
Protocol – indicates which upper-layer protocol, such as TCP or UDP,
receives incoming packets after IP processing has been completed,
eight bits.
Header checksum – helps ensure IP header integrity, 16 bits.
Anatomy of an IP packet:
The IP header consists of the following:
Source address – specifies the sending node IP address, 32 bits.
Destination address – specifies the receiving node IP address, 32
bits.
Padding – extra zeros are added to this field to ensure that the IP
header is always a multiple of 32 bits.
Data – contains upper-layer information, variable length up to 64 Kb.
Routing:
Routing is an OSI Layer 3 function.
The following are the two key functions of a router:
– Routers must maintain routing tables and make sure
other routers know of changes in the network topology.
This function is performed using a routing protocol to
communicate network information with other routers.
– When packets arrive at an interface, the router must use
the routing table to determine where to send them. The
router switches the packets to the appropriate interface,
adds the necessary framing information for the interface,
and then transmits the frame.
A router is a network layer device that uses one or more
routing metrics to determine the optimal path along which
network traffic should be forwarded.
Routing:
Routing metrics are values used in determining the advantage
of one route over another.
Routing protocols use various combinations of metrics for
determining the best path for data.
Routed protocols transport data across a network.
Routing protocols allow routers to choose the best path for
data from source to destination.
A routing protocol functions includes the following:
Provides processes for sharing route information
Allows routers to communicate with other routers to update
and maintain the routing tables
Routing algorithms and metrics:
Metrics can be based on a single characteristic of a path, or
can be calculated based on several characteristics. The
following are the metrics that are most commonly used by
routing protocols:
Hop count:
The number of routers that a packet must travel through
before reaching its destination. Each router the data must
pass through is equal to one hop. A path that has a hop
count of four indicates that data traveling along that path
would have to pass through four routers before reaching its
final destination. If multiple paths are available to a
destination, the path with the least number of hops is
preferred.
Routing algorithms and metrics:
Bandwidth:
The data capacity of a link. Normally, a 10-Mbps Ethernet link is
preferable to a 64-kbps leased line.
Delay:
The length of time required to move a packet along each link from
source to destination. Delay depends on the bandwidth of intermediate
links, the amount of data that can be temporarily stored at each router,
network congestion, and physical distance.
Load:
The amount of activity on a network resource such as a router or a link.
Reliability:
Usually a reference to the error rate of each network link.
Routing Protocols:
Two families of routing protocols are Interior Gateway
Protocols (IGPs) and Exterior Gateway Protocols (EGPs).
IGPs can be further categorized as either distance-vector or
link-state protocols.
Examples of distance-vector protocols include the following:
• Routing Information Protocol (RIP) – The most common
IGP in the Internet, RIP uses hop count as its only routing
metric.
• Interior Gateway Routing Protocol (IGRP) – This IGP
was developed by Cisco to address issues associated with
routing in large networks.
• Enhanced IGRP (EIGRP) – This Cisco-proprietary IGP
includes many of the features of a link-state routing
protocol. Because of this, it has been called a balancedhybrid protocol, but it is really an advanced distance-vector
routing protocol.
Link-state Routing Protocols:
Link-state routing protocols were designed to overcome
limitations of distance vector routing protocols.
Link-state routing protocols respond quickly to network
changes sending trigger updates only when a network
change has occurred.
Link-state routing protocols send periodic updates, known
as link-state refreshes, at longer time intervals, such as
every 30 minutes.
Link-State Routing Protocols:
–IS-IS (Intermediate System-to-Intermediate System )
–OSPF (Open Shortest Path First)
Routing versus switching:
Routing and switching might seem to perform the same function to the
inexperienced observer.
The primary difference is that switching occurs at Layer 2, the data link
layer, of the OSI model and routing occurs at Layer 3.
This distinction means routing and switching use different information in
the process of moving data from source to destination.
Router vs Switches:
Another difference between switched and routed networks is
switched networks do not block broadcasts.
As a result, switches can be overwhelmed by broadcast
storms.
Routers block LAN broadcasts, so a broadcast storm only
affects the broadcast domain from which it originated.
Because routers block broadcasts, routers also provide a
higher level of security and bandwidth control than switches.
Benefits of Subnetting:
• More efficient use of IPs
• Increased address flexibility
• Segments Broadcast domains (smaller)
– Small amount of security
IPs & Subnetting:
For each of the following IPs:
– 172.17.2.175/26
– 101.100.10.89/25
– 219.199.101.140/28
Identify the following:
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–
–
–
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Class
Subnet Mask
# SN bits and # useable Subnets
# Host Bits and # useable IPs
Subnet address for the IP
Subnet Broadcast address
Useable IPs (range)
Major Network Address
Major Broadcast address
IPs & Subnetting - 219.199.101.140/28
Identify the following:
Class:
Subnet Broadcast address (Host Bits = 1):
219.199.101.143
Useable IPs (range):
219.199.101.129  172.17.2.142
Class C
Subnet Mask (Host Bits = 0):
255.255.255.240
Major Network Address:
# SN bits and # useable Subnets:
– 4 Subnet bits
– 24 – 2 = 14
# Host Bits and # useable IPs:
– 4 Host Bits
– 24 – 2 = 14
Subnet address for the IP :
219.199 .101.10001100
255.255.255 .11110000
--------------------------------219.199.101.10000000
219.199.101.128
219.199.101.0
Major Broadcast address:
219.199.101.255
IPs & Subnetting -172.17.2.175/26
Identify the following:
Class:
Class B
Subnet Broadcast address:
172.17.2.191
Subnet Mask:
255.255.255.192
# SN bits and # useable
Subnets:
– 10 Subnet bits
– 210 – 2 = 1022
Useable IPs (range):
172.17.2.129  172.17.2.190
Major Network Address:
172.17.0.0
# Host Bits and # useable IPs:
– 6 Host Bits
– 26 – 2 = 62
Subnet address for the IP:
172. 17 . 2 . 10101111
255.255.255 . 11000000
--------------------------------127.17.2.10000000
172.17.2.128
Major Broadcast address:
172.17.255.255
IPs & Subnetting -101.100.10.89/25
Identify the following:
Class:
Class A
Subnet Broadcast address:
101.100.10.127
Subnet Mask:
255.255.255.128
# SN bits and # useable
Subnets:
– 17 Subnet bits
– 217 – 2 = 131070
Useable IPs (range):
101.100.10.1  172.17.2.126
Major Network Address:
101.0.0.0
# Host Bits and # useable IPs:
– 7 Host Bits
– 27 – 2 = 126
Subnet address for the IP:
101. 100. 10 . 01011001
255.255.255 . 10000000
--------------------------------101.100.10.00000000
101.100.10.0
Major Broadcast address:
101.255.255.255
IPs & Subnetting - 219.199.101.140/28
Identify the following:
Class:
Class C
Subnet Broadcast address:
219.199.101.143
Subnet Mask:
255.255.255.240
# SN bits and # useable
Subnets:
– 4 Subnet bits
– 24 – 2 = 14
Useable IPs (range):
219.199.101.129  172.17.2.142
Major Network Address:
219.199.101.0
# Host Bits and # useable IPs:
– 4 Host Bits
– 24 – 2 = 14
Subnet address for the IP:
219.199 .101.10001100
255.255.255 .11110000
--------------------------------219.199.101.10000000
219.199.101.128
Major Broadcast address:
219.199.101.255
Host Subnet Schemes
The number of lost IP addresses with a Class C network
depends on the number of bits borrowed for subnetting.
Chapter #10
Test