Cisco CCENT/CCNA ICND1 Notes
Chapter 1 - The TCP/IP and OSI Networking Models
TCP/IP Layers
Application
Transport
Network
Data Link
Physical
Same-Layer and Adjacent-Layer Interactions
Same-layer interaction on different
computers
The two computers use a protocol to communicate with
the same layer on another computer. The protocol
defined by each layer uses a header that is transmitted
between the computers to communicate what each
computer wants to do. Header information added by a
layer of the sending computer is processed by the same
layer of the receiving computer.
Adjacent-layer interaction on the
same computer
On a single computer, one layer provides a service to a
higher layer. The software or hardware that implements
the higher layer requests that the lower layer perform the
needed function.
Five Steps of Data Encapsulation: TCP/IP
●
●
Application:
Transport:
●
Network:
●
Data Link:
●
Physical:
Data
TCP, Data
^ Segment
IP, TCP, Data
^ Packet
Data Link, IP, TCP, Data, Data Link
^ Frame
------> Transmit Bits
OSI Reference Model
(All People Seem To Need Data Processing)
Layer
Layer Name
Functional Description
Protocols and
Specifications
Devices
7
Application layer
Interface between
communications software
and any applications that
need to communicate
outside the computer on
which the application
resides
Telnet, HTTP,
FTP, SMTP,
POP3, VoIP,
SNMP
Hosts, firewalls
6
Presentation
layer
Define and negotiate data
formats
"
"
5
Session layer
Defines how to to start,
control, and end
conversations (sessions)
"
"
4
Transport layer
Focuses on data delivery
to another computer
TCP, UDP
"
3
Network layer
Logical addressing, routing
(forwarding), and path
determination
IP
Router
2
Data link layer
Defines rules that
determine when a device
can send data over a
particular medium; also
define format of header
and trailer
Ethernet (IEEE
802.3), HDLC
LAN switch,
wireless access
point, cable
modem, DSL
modem
1
Physical layer
Physical transmission
medium
RJ-45, Ethernet
(IEEE 802.3)
LAN hub, LAN
repeater, cables
OSI Encapsulation
●
PDU: Protocol Data Unit
○ "Layer x PDU"
○ Reference p. 39 for chart
Chapter 2 - Fundamentals of Ethernet LANs
Speed
Common Name
Informal IEEE
Standard Name
Formal IEEE
Standard Name
Cable Type,
Maximum
Length
10 Mbps
Ethernet
10BASE-T
802.3
Copper, 100m
100 Mbps
Fast Ethernet
100BASE-T
802.3u
Copper, 100m
1000 Mbps
Gigabit Ethernet
1000BASE-LX
802.3z
Fiber, 5000m
1000 Mbps
Gigabit Ethernet
1000BASE-T
802.3ab
Copper, 100m
10 Gbps
10 Gig Ethernet
10GBASE-T
802.3an
Copper, 100m
10BASE-T and 100BASE-T Pinouts
●
●
●
Two twisted pairs
Pairs are inserted into pins 1/2 and 3/6
Transmission/Reception
○ Ethernet NIC: Transmits on pins 1 and 2, receives on pins 3 and 6
○ LAN Switch: Receives on pins 1 and 2, transmits on pins 3 and 6
1000BASE-T Pinouts
●
●
●
●
Four twisted pairs
Simultaneously transmit and receive on each wire pair
Pin pairs 1/2, 3/6, 4/5, 7/8
Crossover cable switches 1/2--3/6 and 4/5--7/8
Crossover/Straight-through
●
●
Crossover cable: If the endpoints transmit on the same pin pair
Straight-through cable: If the endpoints transmit on different pin pairs
Transmits on Pins 1,2
Transmits on Pins 3,6
PC NICs
Hubs
Routers
Switches
Wireless Access Point (ethernet interface)
Ethernet Data Link Protocols
●
●
Ethernet Header and Trailer Fields: table on p. 58
MAC addresses are universally unique (unicast Ethernet addresses)
(MAC Addresses)
Organizationally Unique
Identifier (OUI)
Vendor Assigned (NIC Cards,
Interfaces)
Size, in bits
24 Bits
24 Bits
Size, in hex digits
6 Hex Digits
6 Hex Digits
Example
00 60 2F
3A 07 BC
Chapter 3 - Fundamentals of WANs
●
●
●
Leased line: line between LANs leased monthly from a service provider
Customer premises equipment (CPE): customer's router, serial interface card, and
CSU/DSU
CSU/DSU: channel service unit/data service unit, connects with serial connection to
router and telco network; telco's four-wire cable (usually RJ-48 connector) plugs into this
○ Router1<--->CSU<-------------------Telco---------------------->CSU<--->Router2
Data link protocols for leased lines (Layer 2)
●
●
HDLC (High-Level Data Link Control)
○ Point-to-point topology; has an address field but destination is implied
○ Flag, Address, Control (identifies type of L3 packet), FCS (error detection, trailer)
PPP (Point-to-Point Protocol)
Ethernet WANs
●
●
Customer's CPE connects via fiber (Ethernet, i.e. 1000BASE-LX or 1000BASE-ZX) to
service provider's PoP (point of presence); reverse for other end of WAN
Inside of SP: Ethernet emulation or Ethernet over MPLS (EoMPLS)
○ Provides point-to-point connection for two devices over WAN as if a direct fiber
ethernet link existed between them
DSL/Cable
●
●
DSL
○
○
○
Cable
○
○
Telephone line from home runs into DSLAM at telco (DSL Access Multiplexer)
Splits out from DSLAM to voice switch (then PSTN) and internet router
Asymmetric connection (faster down than up)
Splits out data and video on telco side
Asymmetric connection
Chapter 4 - Fundamentals of TCP/IP Transport and Applications
ARP (Address Resolution Protocol): dynamically learns the data link address (MAC address) of
an IP host connected to a LAN
IP host: any device that has at least one interface with an IP address and can send and receive
IP packets
IPv4 Addressing
●
●
Grouping
○ All IP addresses in the same group must not be separated from each other by a
router
○ IP addresses separated from each other by a router must be in different groups
Class A, B, and C IP Networks
Addresses
Class
0
Reserved
1-126
Class A
127
Reserved
128-191
Class B
Unicast
192-223
Class C
Unicast
224-239
Class D
Multicast
240-255
Class E
Experimental
○
○
○
●
Class
Type
Unicast
Class A
■ 126 networks
■ 16,777,214 hosts per network
Class B
■ 16,384 networks
■ 65,534 hosts per network
Class C
■ 2,097,152 networks
■ 254 hosts per network
Network ID (aka network number/network address): one reserved DDN value per
network that identifies the IP network
First Octet Range
Valid Network Numbers
A
1 to 196
1.0.0.0 to 126.0.0.0
B
128 to 191
128.0.0.0 to 191.255.0.0
C
192 to 223
192.0.0.0 to 223.255.255.0
IPv4 Routing
●
●
IPv4 Host Routing Logic
○ Step 1: If the destination IP address is in the same IP subnet as I am, send the
packet directly to that destination host
○ Step 2: Otherwise, send the packet to my default gateway, also known as a
default router (This router has an interface on the same subnet as the host.)
Router Forwarding Logic
○ Step 1: Use the data link Frame Check Sequence (FCS) field to ensure that the
frame had no errors; if errors occurred, discard the frame.
○ Step 2: Assuming that the frame was not discarded at Step 1, discard the old
data link header and trailer, leaving the IP packet
○ Step 3: Compare the IP packet's destination IP address to the routing table, and
find the route that best matches the destination address. This route identifies the
outgoing interface of the router, and possibly the next-hop router IP address.
○ Step 4: Encapsulate the IP packet inside a new data link header and trailer,
appropriate for the outgoing interface, and forward the frame.
IPv4 Routing Protocols
●
●
Routing protocol goals:
○ Learn and fill routing table with a route to each subnet in the internetwork
○ Best route to each subnet if more than one is available
○ Remove invalid routes from routing table
○ Add replacement routes for removed routes
○ Quickly do all of above tasks
○ Prevent routing loops
Steps for learning routes:
○ Step 1: Each router, independent of the routing protocol, adds a route to its
routing table for each subnet directly connected to the router
○ Step 2: Each router's routing protocol tells its neighbours about the routes in its
routing table, including the directly connected routes, and routes learned from
other routers
○ Step 3: After learning a new route from a neighbour, the router's routing protocol
adds a route to its IP routing table, with the next-hop router of that route typically
being the neighbour from which the route was learned
DNS Name Resolution: DNS packet is sent to DNS server; answer is returned to host; IP packet
is forwarded accordingly
Address Resolution Protocol
●
●
●
●
Router needs to know MAC address for neighboring device in order to build frame
ARP: method for any host or router on a LAN to learn the MAC address of another IP
host or router on the same LAN
ARP Request --> ARP Reply
○ Request is a broadcast, reply is a unicast
Results are kept in ARP Cache for a certain amount of time
Chapter 5 - Fundamentals of TCP/IP Transport and Applications
TCP/IP Transport Layer Features
Function
Description
Multiplexing using ports
Function that allows receiving hosts to
choose the correct application for which data
is destined, based on the port number.
Error recovery (reliability)
Process of numbering and acknowledging
data with Sequence and Acknowledgment
header fields.
Flow control using windowing
Process that uses window sizes to protect
buffer space and routing devices from being
overloaded with traffic.
Connection establishment and termination
Process used to initialize port numbers and
Sequence and Acknowledgment fields.
Ordered data transfer and data segmentation
Continuous stream of bytes from an upperlayer process that is "segmented" for
transmission and delivered to upper-layer
processes at the receiving device, with the
bytes in the same order.
Multiplexing: uses "Socket" to determine which application receives data
● Socket includes:
○ IP address
○ Transport protocol
○ Port number
Port Numbers
●
●
Well-known port numbers: used by well-known applications (on servers)
Dynamic port numbers: starting at 1024, assigned to applications by host
TCP Connections
●
●
Connection-oriented protocol: A protocol that requires an exchange of messages before
data transfer begins, or that has a required pre-established correlation between two
endpoints.
○ Three-way handshake (connection establishment): SYN ---> SYN, ACK ---> ACK
○ ACK and FIN (finished) bit used for connection termination
Connectionless protocol: A protocol that does not require an exchange of messages and
that does not require a pre-established correlation between two endpoints.
QoS (Quality of Service): the quality of the data transfer between two applications and in the
network as a whole
●
QoS qualities: bandwidth, delay, jitter, loss