MSIS $4523$network remote

You have assigned addresses to allow communication within the same local subnet, but you still

need to get to other networks. Let$$s extend our snail mail example.

Remote Town$/$Network $($Layer $3)$

Now, what happens if the letter is destined for

someone in another town? $($see envelope$)$ The

postman would again first look at the zip code.

Using the method of comparing the first five

digits of the sender and receiver zip codes, he

would see that these do not match up$.$ At this

point, he doesn$$t care about the physical

address of the recipient but only getting the letter to the correct town. To do this, he must send

the letter to the local mail hub which will then forward mail out of the town. If I have two

loading docks at the postal hub $($one for incoming mail from the local town and one for outgoing

mail beyond my town$),$ my hub connects me to another postal network. I would send mail I want

to go out of my town to the incoming side of the hub so it can route it to the other side. At each

hub along the way, the same process of looking at the zip codes, seeing they are different,

forwarding the mail to the hub, will continue until the letter arrives at its destination town. At

this point, the same process of Local Town$/$Network $($Layer $3)$ will resume from before.

Now, how does this correlate with the networking layer $($layer $3),$ using the example below?

sender IP: $192\mathrm{.}168\mathrm{.}0\mathrm{.}111000000\mathrm{.}10101000\mathrm{.}00000000\mathrm{.}00000001$

destination IP: $139\mathrm{.}78\mathrm{.}8\mathrm{.}10010001011\mathrm{.}01001110\mathrm{.}00001000\mathrm{.}01100100$

subnet mask: $/1611111111111111110000000000000000$

The computer $($acting as the postman$)$ would look at the sender IP and destination IP to see if the

first $16$ bits $($like the first $5$ digits in the zip code$)$ are the same $($i$.$e$.$ the bits in the sender and

destination IPs are the same where they line up with $1$s in the SM$).$ The first $16$ bits are not the

same $($see highlighted$),$ so the computer needs to send the data to its default gateway $($just like

the local mail hub$)$ which will send the data out of the local network. A router $($like a mail hub$)$ is

used to connect multiple networks, so it will have IP settings $($IP$,$ SM$)$ on each of the networks it

connects. A computer wanting to send data to another network would send this data to its default

gateway, which is the IP of the side of router that is on that computer$$s network $($like the

incoming dock for local mail at the mail hub$).$ At each network along the way, the same process

of looking at the IPs, seeing the parts aligning with the number of bits in the SM $($here$,$ the first

$16$ bits$)$ are different, forwarding the packet to the default gateway, will continue until the packet

arrives at its destination network. At this point, the same process of Local Town$/$Network $($Layer

$3)$ will resume.

This demonstrates why layer $3$ is needed. If all traffic was just destined to another computer on

the same network, layer $3$ would not be needed because only the physical $($MAC$)$ address of each

network would need to be known. But like the real world has many towns, the internet has many

networks. If the whole world was all one big LAN, the Internet would slow to a crawl. Think

Jane Doe

$321$ Second Street

Morrison, OK $73061-0002$

Joe Schmo

$345$ Third Street

Glencoe, OK $74032-0005$