When troubleshooting IP networksover Ethernet it helps to understand
how packets travel across the network. Packets use two different
mechanisms to get from point A to Point B, or from the source to the
destination. IP addresses and MAC addresses.
The MAC address is the layer 2 address that represents the specific
hardware that is connected to the wire. The IP address is the layer 3
address that represents the logical identity of the device on the
network.
MAC Address
Most every computer today has some sort of network interface card
(NIC) either built-in or installed on the computer. Every NIC is
created with a hardware number permanently "burned" into it. This
permanent hardware number is known as the MAC (Media Access Control).
MAC addresses are 48 bits in length and are usually displayed as a 12
digit hexadecimal number.
MM:MM:MM:HH:HH:HH
The first 24 bits (or 6 digits) represent the manufacturer of the NIC.
The last 24 bits (6 digits) are a unique identifier that represents
the Host or the card itself. No two MAC identifiers are alike.
IP Address
The IP address is the logical address that is associated with the MAC
for a particular device. IPaddresses (IPv4) are a 32 bit (12 digit)
number representing 4 binary octets.
Both an IP and a MAC are needed for data to travel across an Ethernet network.
The ARP Protocol
When a computer sends data over the network, it first needs to find
which route it must take. Will the packet stay on the network or does
it need to leave the network. The computer first determines this by
comparing the subnet mask to the destination ipaddress.
Once this destination is known, Address Resolution Protocol (ARP) is
used to find the next hop on the network. ARP's job is to basically
discover and associate IPaddresses to the physical MAC.
For a packet that has a destination on another network, ARP is used to
find the MAC of the gateway router. An ARP packet is sent to the
gateway router asking for it's MAC. The router reply's back to the
computer with it's mac address. The computer will then forward the
packet directly to the mac address of the gateway router.
When the gateway router receives the packet it will removeit's mac
address as the destination and replace it with the mac address of the
next hop router. It will also replace the source computer's mac
address with it's own mac address. This happens at each route along
the way until the packet reaches it's destination.
Going Postal - An Example
Let's use the postal service to help explain this. Think of this
process like taking a letter and stuffing it in an envelope. Then take
that envelope and stuff it inanother envelope addressed to the local
post office. The post office then takes out the inner envelope, to
determine the next stop on the way, puts it in another envelope with
the new destination.
how packets travel across the network. Packets use two different
mechanisms to get from point A to Point B, or from the source to the
destination. IP addresses and MAC addresses.
The MAC address is the layer 2 address that represents the specific
hardware that is connected to the wire. The IP address is the layer 3
address that represents the logical identity of the device on the
network.
MAC Address
Most every computer today has some sort of network interface card
(NIC) either built-in or installed on the computer. Every NIC is
created with a hardware number permanently "burned" into it. This
permanent hardware number is known as the MAC (Media Access Control).
MAC addresses are 48 bits in length and are usually displayed as a 12
digit hexadecimal number.
MM:MM:MM:HH:HH:HH
The first 24 bits (or 6 digits) represent the manufacturer of the NIC.
The last 24 bits (6 digits) are a unique identifier that represents
the Host or the card itself. No two MAC identifiers are alike.
IP Address
The IP address is the logical address that is associated with the MAC
for a particular device. IPaddresses (IPv4) are a 32 bit (12 digit)
number representing 4 binary octets.
Both an IP and a MAC are needed for data to travel across an Ethernet network.
The ARP Protocol
When a computer sends data over the network, it first needs to find
which route it must take. Will the packet stay on the network or does
it need to leave the network. The computer first determines this by
comparing the subnet mask to the destination ipaddress.
Once this destination is known, Address Resolution Protocol (ARP) is
used to find the next hop on the network. ARP's job is to basically
discover and associate IPaddresses to the physical MAC.
For a packet that has a destination on another network, ARP is used to
find the MAC of the gateway router. An ARP packet is sent to the
gateway router asking for it's MAC. The router reply's back to the
computer with it's mac address. The computer will then forward the
packet directly to the mac address of the gateway router.
When the gateway router receives the packet it will removeit's mac
address as the destination and replace it with the mac address of the
next hop router. It will also replace the source computer's mac
address with it's own mac address. This happens at each route along
the way until the packet reaches it's destination.
Going Postal - An Example
Let's use the postal service to help explain this. Think of this
process like taking a letter and stuffing it in an envelope. Then take
that envelope and stuff it inanother envelope addressed to the local
post office. The post office then takes out the inner envelope, to
determine the next stop on the way, puts it in another envelope with
the new destination.
No comments:
Post a Comment