CCNA (Part 2)

Preamble

a series of alternating 1s and 0s at the beginning used for synchronization between sender and receiver. 7 bytes long (56 bits). Allows devices to synchronize their receiver clocks so they are ready to receive the packet

SFD (start frame delimiter)

a specific bit sequence indication the start of the frame, helping receivers identify and synchronize with incoming data. Length is 1 byte (8 bits) with a sequence of 10101011. Marks the end of the preamble.

Destination

specifies the intended recipient of the frame (usually MAC address), identifying the device or network adapter

Source

identifies the sender of the frame (usually MAC address), indicating the device or network adapter that transmitted it

MAC (Media Access Control)

6 byte (48-bit) hexadecimal address of the physical device

Type/length

specifies either the type of the payload data or the length of the payload, depending on the Ethernet frame type used.

1500 or less

type/length field value that signifies the LENGTH of the encapsulated packet (in bytes)

1536 or greater

type/length field value that signifies the TYPE of the encapsulated packet (usually IPv4 or IPv6), and the length is determined via other methods

0x0800

value used in type/length field to signify the type is IPv4

0x86DD

value used in type/length field to signify the type is IPv6

FCS (frame check sequence)

a field found in the Ethernet frame's trailer and is used for error checking, containing a value calculated based on the frame's contents to ensure its integrity during transmission. Uses the CRC (cyclic redundancy check) 4 bytes

CRC (cyclic redundancy check)

used in FCS. A type of error-checking code used to detect errors in the frame's data, ensuring data integrity during transmission

OUI (organizationally unique identifier)

the first 3 bytes or half of a MAC address. Assigned to the company making the device

Unicast frame

an Ethernet frame that is sent from one device to one specific destination device on a network

MAC address table

a database on a network switch that maps MAC addresses to the ports on which devices are connected, allowing the switch to forward frames to the correct destination

Dynamic MAC address

a MAC address that is automatically learned by a network device, such as a switch, and is not manually configured

Unknown unicast frame

an Ethernet frame that is addressed to a specific destination MAC address but cannot be forwarded by a switch because the switch does not have an entry for that MAC address in its MAC address table so it is flooded on all interfaces but the interface that the packet was received on

Known unicast frame

an Ethernet frame that is addressed to a specific destination MAC address and can be forwarded by a switch because the switch has an entry for that MAC address in its MAC address table

46 bytes

minimum payload packet size

Padding

added if the payload does not reach the minimum size (46 bytes). Represented in 0000s

IP packet

found on Ethernet frame and includes a source and destination IP address

ARP (Address resolution protocol)

protocol used in TCP/IP networks to map IP addresses to MAC addresses, allowing devices to communicate at the data link layer

ARP request

a broadcast sent to all host on the network that is used to find out the MAC address of the destination device. Uses FFFF.FFFF.FFFF as a placeholder for the unknown MAC address of the destination device

ARP reply

a unicast that is sent only to one host (host that sent the request) to discover the destinations MAC address

arp -a

command with option used in Windows, macOS, and Linux to display the ARP table. Include the Internet address(IP address), physical address(MAC address), and type (static or dynamic)

show arp

command used in privileged exec mode to show ARP table

show mac address-table

command to view the MAC address table on Cisco switch. Includes VLAN, MAC Address, Type (Static/Dynamic), and Ports(interface)

aging

refers to the process of removing entries from the table after a certain period of inactivity (5 minutes), ensuring that the table remains up to date with the device connected to the network

clear mac address-table dynamic

command to clear all dynamic mac addresses from mac address table

clear mac address-table dynamic address (mac address)

command to clear a specific dynamic mac address from mac address table

clear mac address-table dynamic interface (interface-id)

command to clear a specific interface from mac address table

32 bits (4 bytes)

length of IPv4 address

128 64 32 16 8 4 2 1

binary octet factors from left to right

0-127

first octet numeric range of IPv4 class A

128-191

first octet numeric range of IPv4 class B

192-223

first octet number range of IPv4 class C

224-239

first octet number range of IPv4 class D (multicast)

240-255

first octet number range of IPv4 class E (reserved for experimental uses)

loopback address

special IP addresses (127.0.0.0 to 127.255.255.255) reserved for a device to refer to itself, allowing testing of network functions without sending data over a physical network

/8

prefix length (network address) of IPv4 class A

/16

prefix length (network address) of IPv4 class B

/24

prefix length (network address) of IPv4 class C

2^n -2

the formula used to calculate the amount of addresses per IPv4 class where n is the length in bits of the host portion of the IPv4 address

Netmask

a 32-bit binary number used in IP networking to divide an IP address into network and host portions, helping routers determine the network to which an IP address belongs

255.0.0.0

IPv4 class A netmask

255.255.0.0

IPv4 class B netmask

255.255.255.0

IPv4 class C netmask

Network address

host portion of the address is all 0s and signifies the address of the entire network

Broadcast address

host portion of the address is all 1s and is used to send a broadcast to all devices on that network (255)

show ip interface brief

command on Cisco routers (privileged exec mode) that displays a summary of the IP addresses assigned to each interface, along with their status and protocol information (Interface, IP-Address, Ok?, Method, Status (layer 1), Protocol (layer 2)

status

shown in "show ip interface brief" command in Cisco routers and by default is administratively down (layer 1). This tells you if the interface is turned on or off

Protocol

shown in "show ip interface brief" command in Cisco routers (layer 2) tells you if the Ethernet is working properly

interface (interfacetype interface-ID)

command used in global configuration mode to bring you to the interface configuration mode

R1(config-if)#

prompt is Cisco routers that tells you are in interface configuration mode

ip address (ip-address)(subnetmask)

command in interface configuration mode that allows you to set the ip address and subnetmask of an interface on a router

no shutdown

command used in Cisco routers that turns on an interface from the default down

show interface (interface-ID)

command used in privileged exec mode that will show you are more detailed view of the specify interface ID

show interface description

command used in privileged exec mode that will show you all the interfaces status, protocol, and description within Cisco routers

description (description of interface)

command used in interface config mode that will allow you to create a description of the interface (this is optional)