LAN without a router: How does it work?

wellnowwhat

I'm currently reading through a Network+ book and the author has given several examples of LANs using only switches or hubs. I've always had a router involved, so I'm trying to make sense of how that would work. Would you have to assign static IP addresses within the same range for the machines to communicate? What about DNS resolution?

Just trying to get a sense as a beginner.

phoeneous

Google these terms: OSI Model, Layer 2, MAC Address, ARP, Subnet

The short story is, devices can communicate with each other at layer 2 while in the same subnet through the magic of arp and the mac address table.

wellnowwhat

So, then, is it the job of a switch to hold that MAC address table and when it receives a request from ARP then subsequently look at its table to determine where the data is supposed to go? And what about a hub? From what I can tell hubs just "flood" the network with packets and any computers listening for those packets receive them..

Sorry if these are very simplistic questions.

ptilsen

Keep reading your book. What you'll come to understand is that your "router" is so much more than a router.

For now, the short answer is the routers are only needed to connect networks. Your network works independently of your router.

bermovick

wellnowwhat wrote: »

Would you have to assign static IP addresses within the same range for the machines to communicate?

Yeah, pretty much.

wellnowwhat wrote: »

What about DNS resolution?

For a standalone LAN, DNS likely wouldn't be necessary. There'd be WINS for some hostname resolution or something similar.

Keep in mind too, frequently in these books, you aren't going to be seeing a full network - just something specifically designed to show you whatever the current subject is - so if the subject is layer 2, it's only going to show you a single layer 2 topology.

wellnowwhat

So my "router" likely has a switch or hub built into it, and any local traffic goes through that portion, but if I, say, need to get to the web, that's when the "routing" happens and the two networks communicate?

It's starting to become clearer as I think about it. Thanks.

bermovick

Bingo

drkat

wellnowwhat wrote: »

So my "router" likely has a switch or hub built into it, and any local traffic goes through that portion, but if I, say, need to get to the web, that's when the "routing" happens and the two networks communicate?

It's starting to become clearer as I think about it. Thanks.

Pretty much ... Just think of it this way.. a router connects networks / is the gateway - Just keep reading it'll make sense.

Iristheangel

When your computer wants to communicate with a device on the same subnet, it needs to know it's Mac address so it sends out an ARP request that basically says "Hey, this is 192.168.1.4, whats the Mac address for 192.168.1.5?" (Read about it in more detail here: How ARP Works). Remember: Mac address fall within the layer 2 of the OSI model. After the computer has the Mac address for that destination IP address, it saves it in a table and uses it for future communication with that host. If the computer needs to send data, it will always include the destination Mac address in every packet.

Without complicating it by bringing up all the different devices out there, a regular switch is a layer 2 device. Any device on the same subnet can communicate with each other using a switch. The switch keeps track of who is sending data to who by keeping a Mac table. If the switch gets a packet with a destination Mac address it does not recognize, it'll send it out ever port on the switch and when it gets a response, it'll add that Mac address to it's table. Basically, switches ignore the IP addresses and just goes based on the Mac address.

A router is a layer 3 device. It operates using IP addresses and it's so you can communicate between subnets or with other networks. If you send a packet to a host on another network/subnet, your computer will send the packet to the router and the router will look at it's routing table (which can be statically configured or can be learned through routing protocols, but that's a whole different conversation) and then send the packet to the next router in line to that destination.

So you see, you can have a fully functional LAN without a router as long as you never need to route packets to a destination outside that subnet. In most cases, there won't be a need to perform DNS queries on a local isolated network unless you wanted to set up a test DNS server and create records pointing to other hosts in the network. You can also have statically assigned IP address for every host on the network or you could put a DHCP server on the network that would assign addresses automatically. You don't need a router for DHCP.

Anyways, hopefully that cleared things up for you! What book are you using for Network+ if I may ask?

wellnowwhat

Thanks, Iris (and everyone else, too). That definitely helps.

I've been reading the Todd Lammle Network+ book, but it's hard for me to follow his writing style and structure, so I may switch to another book when I get a chance.

Iristheangel

I used the Mike Myers book years ago and I really liked this writing style. If you look up his book on Amazon, his Network+ books have almost 5 stars all across the board.

Here's a good free Network+ video series as well: Professor Messer's Free CompTIA Network+ Certification Training Course | Professor Messer - CompTIA A+, CompTIA Network+, Certification Training
But never just watch the videos and assume you know the material. Every video series I've ever used for certifications always tend to leave about 30-40% of the relevant details out, but they're still nice to use as a reinforcement to what you're currently reading.

Hope that helps

wellnowwhat

I'd love to pick up the Meyers book, but it's the most expensive of the bunch (plus, no Kindle edition :P).

I downloaded a sample of the Kevin Wallace book and it looks promising, and it's a bit cheaper than the Meyers book. Heck, I may just buy both when I have the cash.

drkat

wellnowwhat wrote: »

Thanks, Iris (and everyone else, too). That definitely helps.

I've been reading the Todd Lammle Network+ book, but it's hard for me to follow his writing style and structure, so I may switch to another book when I get a chance.

CBT Nuggets Network+ isnt too bad.. I'd also look at Professor Messer

I'm a big fan of Exam Cram http://www.amazon.com/CompTIA-Network-N10-005-Authorized-Paperback/dp/B0084IT44I/ref=sr_1_1?s=books&ie=UTF8&qid=1338006187&sr=1-1

The reason being: If you're like me you want to get right to it and skip all the semantics.. I dont like useless filler, just give me the material and if I have questions I'll either ask someone or google it. Nothing is worse than staring at the thickness of a 500 page book and feeling overwhelmed. I think that size and content of the exam cram series gives you a "Just Enough" style of preparation and you can fill in the details later; because let's face it? Experience gained is goin to be your best teacher

I cant stress enough how much you forget when reading those big books (if you're like me) until you're fixing problems and actually using the information, YMMV due to the environment you work in and what skils you will be utilizing.

ProfessorMesser

Iristheangel wrote: »

But never just watch the videos and assume you know the material. Every video series I've ever used for certifications always tend to leave about 30-40% of the relevant details out, but they're still nice to use as a reinforcement to what you're currently reading.

I couldn't agree more. Because of the nature of the medium, it's impossible for a video to have the same "density" of information as a book. Use the videos, but make sure you have a good book for the detailed information you won't get in a video. I'd also recommend that you get a good Q&A book or sample exam, just to round things out.

Iristheangel

ProfessorMesser? Wow. Nice to see you on here. Nice job with the video series. They were easy to follow along when I was studying for my Network+

sratakhin

Windows can also be in a network without a router and static IPs by using APIPA.
More details: What's APIPA?

wellnowwhat

If you're like me you want to get right to it and skip all the semantics.. I dont like useless filler, just give me the material and if I have questions I'll either ask someone or google it.

I know exactly what you mean! I understand that for some people the context and history are interesting, but when I'm trying to learn something technical it's hard to push past all of the filler and just get to the "raw" information about what I'm trying to learn. I'd rather learn from a more concise source than one that sprinkles in a lot of anecdotes and the like.

I cant stress enough how much you forget when reading those big books (if you're like me) until you're fixing problems and actually using the information, YMMV due to the environment you work in and what skils you will be utilizing.

I definitely know what you're talking about. When I took my Apple certification tests, about 30-40% of the material was just proselytizing about how great Apple's stuff works. I don't need to be "sold," I just want to learn the material. All that other stuff just serves to confuse and over-complicate the material.

drkat

Dude I'm doing a video learning series for the CCNA Voice and the first 2 hours is cisco marketing on how awesome their call manager is I dont think I'm past Module 2.... I just ordered the 640-461 Official Exam Guide by Valentine/Ciora (For offline study.. sitting in front of the computer to study when you sit at it all day leads to... burn out...) who are both awesome authors.... i mean real man crush here. I used Valentines CCNA Exam Cram and Ciora's CBT Nuggets so yeah... **blushes** anyway...


Good luck in your studies.

phoeneous

wellnowwhat wrote: »

So, then, is it the job of a switch to hold that MAC address table and when it receives a request from ARP then subsequently look at its table to determine where the data is supposed to go? And what about a hub? From what I can tell hubs just "flood" the network with packets and any computers listening for those packets receive them.. Sorry if these are very simplistic questions.

For reference, this is what the mac address table looks like on a cisco switch but the same basic premise exists on all switches.

drkat

The hub is a physical layer device and just repeats the signal put into it and floods it out of every port... these are known as frames

The switch on the other hand is a layer 2 device which does not just flood everything.

Frame comes into the switch port the first thing the switch does is record the source mac address of the device sending the frame, it then looks at the destination and says "Do I know where this is?" so it checks it's address table, if it does know where the destination is it will simply forward the frame out of that port, If however it does not know it - the switch will forward the frame out of every port except the one it came in on and will wait for acknowledgement and then update it's mac table

KenC

OP, I think you are getting the hang of it now - the typical home router has a built in switch (the one device can provide many functions such as modem, WAP). Routing only takes place when communications go outside your own LAN.

sratakhin wrote: »

Windows can also be in a network without a router and static IPs by using APIPA.
More details: What's APIPA?

I'm glad someone mentioned it.

Unlike some other contributors here, I would advise you to have a good book that covers everything in depth. It is always advisable to have a good reference book that you can refer to should (or typically when) the need arise(s). I myself used the Mike Meyers Network+ book and found it to be very good (more enjoyable read than the A+ but that could be to do with the subject matter). I never tire of recommending Professor Messer, but only as an additional resource (his Windows 7 70-680 course is great).

Also, if you are planning to do the CCNA it might be a good idea to stick with Lammle's book on Network+ so you can get used to the way he presents the subject. It could stand to you for the CCNA as his book seems to be the most recommended.