🔒 Networking 101

[mortal tendon]

Learning basic networking for an interview

[summer aspen]

@mortal tendon

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[mortal tendon]

@pastel idol

[pastel idol]

so lets recap

10.0.0.1 - 10.0.0.255 network mask 255.255.255.0

the network mask is there to stop any unwanted ip addresses

so it will let me do ping 10.0.0.10

because that is within the network mask range

is that clear?

[mortal tendon]

ok...

im going to have to look up how to do that but for now i understand the concept

[pastel idol]

yes, details matter when you want to do something, concept is what you learn from

so what if i try to access an ip outside of this range

10.0.1.10

[mortal tendon]

a guess is that you wont get any response

[pastel idol]

well, the network mask tells the computer that it is out of range

so what happends when an ip is out of range?

your network card will just send the traffic to the gateway instead

thats it, thats how all network traffic works

if it is out of range, send the traffic to the gateway

[mortal tendon]

this is new to me

ok...

so a subnet is essentially a mask?

[pastel idol]

so if you increase your range, changing the mask, you can access a larger or smaller part of the network

[pastel idol]

no a subnet is this

10.0.A.0

its another part of the network, where the A is

and i should have called it C instead, since its common to name it A.B.C.x

[mortal tendon]

my current understanding is that e.g. 10.0.21.x would be one subnet, and 11.2.32.x would be another

or would it be more like 10.0.1.x and 10.0.2.x?

[pastel idol]

yes, but you need a very wide mask for those to be connected, and you have to own all ips in the range

10.x.x.x is a prrivaate range, so you own them all

[pastel idol]

two C nets next to each other is quite easy to access

you only have to adjust the mask a tiny bit

lets make an example

[mortal tendon]

so A is wider than B, which is wider than C, and D is the specific machine?

[pastel idol]

correct

but its just a human representation

the computer just uses an integer

10.0.0.0 with the subnet of 255.255.0.0

that goes from the range 10.0.0.1 - 10.0.255.255

[mortal tendon]

10.0.0.0 is the gateway?

[pastel idol]

well, might have to exclude the broadcast ip 10.0.255.255

[pastel idol]

no, thats invalid, the gateway is just one ip in this range, normally the router

[mortal tendon]

hmmm

[pastel idol]

lets make another example

[mortal tendon]

so realistically it would be 10.0.0.1

[pastel idol]

it can be any, conventions are that the first is the gateway in most networks

[mortal tendon]

ok

[pastel idol]

10.0.0.0 with the subnet 255.255.254.0

so this is one larger than the first example with only 255 addresses

so it lets trough two C nets

10.0.0.1 - 10.0.1.254

[mortal tendon]

ok...

[pastel idol]

and if you put the subnet to 255.255.252.0 you get what i have

10.0.0.1 - 10.0.3.254

[mortal tendon]

252?

[pastel idol]

10.0.0.0 /22 for short

[pastel idol]

yes, not all numbers are valid

[mortal tendon]

so a wider range doesnt have to be A and B

[pastel idol]

its just how the binary bits line up

[mortal tendon]

it can be just part of it

[jagged grail]

What's the role you applied for if I may ask?

[mortal tendon]

it's not actually related to networking

[pastel idol]

correct, you have quite the granularity

[mortal tendon]

im trying to understand their architecture

[pastel idol]

lets say your company needs 4000 ipaddresses

[mortal tendon]

my educated guess as to what my role is is: there is a fastapi server accessing data from a psql instance on the "usage" subnet

[pastel idol]

wwell, the closest to that is 4096

255.255.240.0 using 10.0.0.1 - 10.0.15.254

or /20 for short

[mortal tendon]

hmm

[pastel idol]

10.0.0.0    10.0.0.1 - 10.0.15.254    10.0.15.255
10.0.16.0    10.0.16.1 - 10.0.31.254    10.0.31.255
10.0.32.0    10.0.32.1 - 10.0.47.254    10.0.47.255
10.0.48.0    10.0.48.1 - 10.0.63.254    10.0.63.255
10.0.64.0    10.0.64.1 - 10.0.79.254    10.0.79.255
10.0.80.0    10.0.80.1 - 10.0.95.254    10.0.95.255
10.0.96.0    10.0.96.1 - 10.0.111.254    10.0.111.255
10.0.112.0    10.0.112.1 - 10.0.127.254    10.0.127.255
10.0.128.0    10.0.128.1 - 10.0.143.254    10.0.143.255
10.0.144.0    10.0.144.1 - 10.0.159.254    10.0.159.255
10.0.160.0    10.0.160.1 - 10.0.175.254    10.0.175.255
10.0.176.0    10.0.176.1 - 10.0.191.254    10.0.191.255
10.0.192.0    10.0.192.1 - 10.0.207.254    10.0.207.255
10.0.208.0    10.0.208.1 - 10.0.223.254    10.0.223.255
10.0.224.0    10.0.224.1 - 10.0.239.254    10.0.239.255
10.0.240.0    10.0.240.1 - 10.0.255.254    10.0.255.255

[jagged grail]

I wonder If I have to understand deep networking as well for ai autonomous engineering. Anyways good luck.

[marsh rain]

Is that what you’re meant to do? I’ve always thought it’s just easier to have it be 255.255.0.0 and then Just not use the rest of the IPs

[mortal tendon]

feel free to answer the question, i would stand to learn something too @pastel idol

[pastel idol]

when you increase your network mask, you will end up having more address to check before traffic is sent to your gateway

for a private network, it will not matter much, because nobody uses those ipaddresses

255.255.0.0 is /16

[marsh rain]

255.255.240.0 is /20 right? Or is my maths wrong

[pastel idol]

thats 65536 ipaddresses

[pastel idol]

correct

[crimson sable]

I would be teaching this more from IP routing than "netmasks" and class A,B,C myself.

I guess you're there one you start talking about the local router.

[pastel idol]

thats how we started

[marsh rain]

Wait @mortal tendon how much do you know about networking? I don’t know that much but I’m just wondering if you actually want genuine 101, like intro to networking

[pastel idol]

but networking is not easy, its just simple

[crimson sable]

Ah. I skimmed ,but may have missed something.

[pastel idol]

makes it deviously hard to understand

[mortal tendon]

barely any

[pastel idol]

we started in another channel

[marsh rain]

What do you know already? Just like any facts or anything you know at all

[crimson sable]

The basics are easy to explain. Local IP addr, just drop it onto the local wire with the MAC addr. Not a local address? Send it to the router for it to forward.

Everything from there on is mechanics and fiddliness.

[mortal tendon]

i know that you connect to another machine with an ip address and a port for some specific program

[pastel idol]

if the ip is not within the network mask, it gets forwarded to the gateway

[mortal tendon]

so this could be an address on the other subnet?

[crimson sable]

Ok. So the IP gets the packet to the machine. The port says which service on the machine you're trying to use.

[pastel idol]

yes, within the rules of available subnets

i can reach three other subnets within my own local network

[mortal tendon]

how would the gateway know which machine in the other subnet to talk to?

[crimson sable]

Because its routing table says so. It knows the directly connected networks (the IP ranges in use on its netword cards). And it will have a default gateway for other addresses not directly connected.

[pastel idol]

the package of data you send is labled with the information it needs

[crimson sable]

(And potentially additional routes, but ignore for now.)

[mortal tendon]

hmmmmmmmmmmmmm

not getting it

[crimson sable]

Let me show you my network, if you'd like a concrete example.

[mortal tendon]

that would be nice

[marsh rain]

Are you asking how one device on a subnet communicates with a device on a separate subnet?

[mortal tendon]

yea

i mean, yes, it's an IP

but like

yknow

[pastel idol]

think about how does 10.0.0.2 and 10.0.0.3 communicate

my network card sends a package addressed to 3

[mortal tendon]

ok

[pastel idol]

the network checks, is this ip close to me? yes/no answer is yes.. and i just send it to that machine instantly

[crimson sable]

Here at home there's a local network. It is 172.16.3.0/24. All the machines on the local net are directly addressable here.
My Mac's wifi IP addr is 172.16.3.99. Because this is a /24 network, the network is 172.16.3 and the local part if .99. Neatly aligned on bytes, because it's easy to think about - that's why we chose /24 - aligned on a byte boundary.

[pastel idol]

im simplifying alot here

[jagged grail]

what if it's a no?

[crimson sable]

So to send a packet to eg the home server, whose address is 172.16.3.20, I split that on my netmask. It has the same network (172.16.3.0) and .20 for the local part. SO I know it's on my own network.

[mortal tendon]

ok

that makes sense

[pastel idol]

traffic is sent to the gateway

[mortal tendon]

that's like how i can use my phone to access my local webserver

[crimson sable]

SO I can drop a packet on my local LAN for the home server. (The ethernet packets are labelled with my MAC address - my netwrk card's MAC address and the server's MAC address.) Because the dst ethernet address is the server it picks it up. And checks the IP address is for it. Ok, packet received.

There's a table mapping IP address to thernet (MAC) address for the local network.

[mortal tendon]

pardon me for the really dumb question, even though @pastel idol explained earlier, i still dont understand what the /24 means

[marsh rain]

what’s 255 in binary?

[crimson sable]

It means that the first 24 bits are the network part of the IP address and the last 8 bits are the local part.

[mortal tendon]

uh 11111111?

[marsh rain]

How many bits does it take to represent 255 then?

[pastel idol]

its a mask shorthand, everything about a network mask is binary

[mortal tendon]

8

[crimson sable]

The netmask is 24 1s. So to get the network part we mask off the top 24 bits, thus 172.16.3.0 for mmy LAN.

[mortal tendon]

oh 3 * 8 = 24

[marsh rain]

So what’s 255.255.255.0?

[pastel idol]

see.. binary

[mortal tendon]

so A.B.C

[crimson sable]

A class A is a /8 - just the first 8 bits are the network.

B is /16 and C is /24

[mortal tendon]

i see

[crimson sable]

These days we don't use those classes, as such, very much.

[mortal tendon]

a typical home network would be a /24

[marsh rain]

/24 means you are using 24 bits to represent the network

[pastel idol]

the network vidualisation is just for humans. its really just a number..

[marsh rain]

Mhm

[pastel idol]

yes, very very common

[jagged grail]

"203.0.113" indicates the Class C network, and "112" indicates the device.
something like this right?

[crimson sable]

netmasks which are /8 or /16 or /24 are common because you can just pick the network and local part of of the IP address trivially by eye.

They can be at abn arbitrary offset.

[mortal tendon]

you could thus have something like /24 for one subnet, and (theoretically) a /23 for a network encompassing the 2 subnets

[crimson sable]

So, my local LAN. My firewall/router is at 172.16.3.1. It is common to put the router at .1. Not required, just easy to remember.

[pastel idol]

!e

import ipaddress
ip = ipaddress.IPv4Address('10.0.0.1')
print(ip)
print(int(ip))
print(bin(int(ip)))

[summer aspen]

:white_check_mark: Your 3.14 eval job has completed with return code 0.

001 | 10.0.0.1
002 | 167772161
003 | 0b1010000000000000000000000001

[mortal tendon]

i see

[pastel idol]

yay.. i did it

[mortal tendon]

i think earlier my question was

there is one machine hosting the main db on the main subnet

and a machine hosting the scoped db on the "usage" subnet

[crimson sable]

If I want to send a packet to eg cloudflare's DNS address, 1.1.1.1.
I mask of the /24 for my local LAN and see that 1.1.1.0 is not my network.
So I send a packet via my firewal/router.

[mortal tendon]

how would you take a subset of a data from the main machine and replicate it on the other

(psql instance)

[mortal tendon]

ahhh

that clears things up a little bit

[marsh rain]

(I might be wrong so someone correct me if I am) your subnets will all have a Default Gateway, a way out of the subnet, like a router, and they would communicate via that, just like you do on your regular network with your router communicating with the internet

[crimson sable]

That means:

• an ethernet packet with a source MAC addr of my machine and a dest MAC address of the firewall's MAC address
• an IP address of my machine (172.16.3.99) and a destination IP of 1.1.1.1 (where the packet is going

The dest ethernet/MAC addresses mean that the firewall picks up the packet.

[pastel idol]

and the firewall/router is the ip called the gateway

[mortal tendon]

the router is the gateway?

[pastel idol]

thats how all networks work, if the ip is not in range, traffic is sent to the gateway

[pastel idol]

yes, in this example

[crimson sable]

The router/gateway looks at my packet, discovers it doesn't match any of the networks it is directly connected to, and sends it to its gateway. My ISP, upstream.

[marsh rain]

A router is very commonly the Gateway, allowing things in and out of a network

[marsh rain]

That’s basically the router’s entire job

[pastel idol]

i said earlier, its very simple... but desepticly hard

[crimson sable]

If the packet was for anther network connected to the router, it would send the packet out on that network. Same source and destination IP addresses (mine and the dest) but a src MAC of the firewall and dst MAC of the machine on the locally connected network.

[mortal tendon]

so 2 subnets, 2 gateways, when i try to communicate with some IP outside of this subnet mask, but within range of the other subnet, it gets sent to my subnet's gateway and then through the other subnet's gateway to the other machine

[crimson sable]

Sounds right.

[mortal tendon]

how do i know what ip to send to?

[crimson sable]

Each router makes its own local decisions.

[marsh rain]

Yup! Subnets are blocked off from eachother so they have to communicate as if they are 2 regular separate networks

[crimson sable]

You look up the DNS name and get an IP address. ANd use that.

[marsh rain]

Do you know what IP addresses on a subnet might look like?

[mortal tendon]

in this case it's an intranet so i dont think they use a dns

[crimson sable]

They will have local DNS.

[mortal tendon]

hmm

[crimson sable]

Like router, nonlocal DNS asks upstream.

[mortal tendon]

no, i might be able to by skimming for an IP address in the codebase

their codebase is GPL-ed

[crimson sable]

Or a name. Some machines have useful names in the /etc/hosts file.
Regardless, somewhere you obtain an IP address to stick on your packet.

(I need to set up and kick off some rice.)

[mortal tendon]

hmm

so now the question becomes: how do 2 psql instances on 2 machines communicate with each other

im not sure what the "usage" subnet uses for its db, but the master db uses psql

[pastel idol]

in details or just in this context?

they know the IP and they sent traffic to that ip

[mortal tendon]

in this context?

[mortal tendon]

yes

[pastel idol]

this is me trying to trace the path my packages takes from me, to github.com

but not all things in the middle are meant for me to ping

[mortal tendon]

my guess is that you would have a ""backend"" on the master subnet machine that serves a machine on the usage subnet

and that machine just writes the data into its psql instance

i have no idea though

[pastel idol]

its just network traffic

one package is about 1400ish bytes large

[mortal tendon]

there is another question that is not related to networking

[pastel idol]

ask away

[mortal tendon]

suppose i have 2 research teams that want to get some dataset. i want to allow one of the team to access their data while i am transferring some data from the master db to this usage db for the other team, i would probably need a multitenant setup on psql to avoid read-write-blocking issues right?

or should i just do 2 separate psql instances?

furthermore, since these operations are all IO bound, my guess is that i should have no problems at all using python to orchestrate these

[pastel idol]

postgress is well supported for multiple users. and yes no problem using python

[mortal tendon]

my intuition tells me that it would be wiser to avoid sharing db since there are different data privacy scope that different teams requested for

just an intuition, not even an educated guess

[pastel idol]

yes, i think so as well, i would run two instances

[mortal tendon]

thank you

i will come back with more questions

[pastel idol]

but there is nothing wrong with either

you have to handle authorization which ever way you go

[mortal tendon]

hmm

[pastel idol]

that means, what a specific user has access to

at work we have both, multiple instances and multitennant setup

and we use python to orchestrate data between instances

[mortal tendon]

sounds prudent

[pastel idol]

its a mess really

[mortal tendon]

i wouldnt believe it

[pastel idol]

but a mess out of nessessity

we are working on fixing it, but its a live system so we cant just take things down

[mortal tendon]

do you think it's good idea to try to replicate a setup that they have just to have an idea of what questions might be asked during the interview?

or like to show off

perhaps it's behavioral though

[pastel idol]

oh yes. that would be a great thing to do

[mortal tendon]

i think... i am to do it within the time

or at least, i know the steps to

!close

[summer aspen]

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