Only two hosts are available with IP addresses
192.168.1.2
and
192.168.1.3
What will be the routing table entries, if one has to setup?
What will be the Next Hop if only one interface is there on each machine.
Data can be transferred to and from both the machines.
Basically, my doubt is what will be the next hop in each case
If the packet has to be delivered to some other network (then i think it will be the IP address of the router connecting to it)
If the packet has to be delivered locally (same subnet) what should be the next hop IP
If the packet has arrived on the host, and it has to be given to upper layers (since it has to see once the routing table, what will be the entries in this case)
All entries in the routing table are static.
Thanks
If you have the standard network mask of 255.255.255.0 for those IP addresses, then no you don't need a routing table, or a default gateway since both machines are in the same network.
A routing table is basically a set of directions for how to get to another network. It says that if you want to go to 12.34.56.78 then go out of x network card, and to go 192.168.1.x (presuming you are on the 192.168.1.0 network)
A default network basically says if you're not on this network, and there's not a more precise route i.e. 12.34.56.78 above. then just go through a particular network address.
A local network works through broadcasting, machine A will send a broadcast packet to MAC (hardward) address FF:FF:FF:FF:FF:FF which will be accepted by all traffic on the network. (that isn't separated by a router). The broadcast packet basically says 'who owns IP Address x.x.x.x'
When the machine that owns that address receives that packet it sends a reply back saying 'I do'
Machine A can then start sending packets directly to MAC address of that machine, and Machine can reply to the MAC address that sent them.
From this you can understand why routing becomes important, once you start having a large number of hosts on the network, the network can become flooded just from the sheer volume of ARP requests that are bouncing around it. By splitting the address space into separate address blocks you can limit the number of hosts, and number of ARP requests that the network needs to broadcast.
To understand how this works at a lower level, you need to understand how IP addresses and subnet masks are used to determine network membership. Internally an IP Address and network mask are represented as binary numbers. The subnet mask will define the split between the network and the subnet. (it uses a Logical AND obtain the network id)
Address: 11000000 10101000 00000010 00000010
Subnet Mask: 11111111 11111111 11111111 00000000
Once it has the network id it can compare that to the address it is trying to send (using the same logic) to and determine what to do with it.
The logic for that goes something like this
If a packet's Network id matches a local network id send it directly, without routing.
If a packet's network id matches a specific route in a routing table. for instance if the packet is 10.1.1.1, and there is a route for 10.1.0.0/255.255.0.0 then forward the packet to the route marked in that routing entry.
If a packet's network id doesn't match a specific route, and there is a default gateway specified, then forward it there.
If there is no specific route, and no default gateway, return a failed send.
P.S. this is very simplified, in reality there are additional nuances!
Related
When we establish a TCP connection from PC1 to Server and send data through this connection, how does the Router know to which of the two PCs (PC1 and PC2) should it communicate on the way back from Server to PC1?
And
How does ping (ICMP) know to which internal node it should send the answer?
NAT (Network Address Translation) is stateful. It creates a translation table that has the layer-3 and layer-4 protocols and addresses. By looking up the return traffic addresses in the translation table, the NAT process can determine which inside addresses should be placed in the packet.
Edit:
Per the edit to your question, asking about ICMP (it is very bad form to change the question in order to ask a different question because it can invalidate the perfectly acceptable answers already given):
It is all the same as TCP or UDP. NAT creates a state table that is dynamically built as traffic passes from inside to outside. NAPT will allow you to overload a single IP address with traffic from multiple inside addresses, and it will translate the return traffic by looking up where to send it in its state table.
With NAPT, besides looking at and translating the IP address, NAPT looks at the layer-4 protocol (TCP, UDP, ICMP) and translates the layer-4 addresses (TCP or UDP port numbers or ICMP identifies), too, storing the translations in its translation table. When return traffic is destined to a particular layer-3 and layer-4 address combination, from a particular layer-3 and layer-4 address combination, the NAPT process finds that in its translation table, and it can see where to send the traffic on the inside.
NAT is very resource intensive, and it breaks the IP paradigm of end-to-end connectivity, where every host is uniquely identified by its own IP address, which is why it is called a kludge (or worse). NAT was developed to extend the life of IPv4 until IPv6, with its nearly unlimited addressing, can become ubiquitous.
RFC 2663, IP Network Address Translator (NAT) Terminology and Considerations:
4.1.2. Network Address Port Translation (NAPT)
NAPT extends the notion of translation one step further by also
translating transport identifier (e.g., TCP and UDP port numbers, ICMP
query identifiers). This allows the transport identifiers of a number
of private hosts to be multiplexed into the transport identifiers of a
single external address. NAPT allows a set of hosts to share a single
external address. Note that NAPT can be combined with Basic NAT so
that a pool of external addresses are used in conjunction with port
translation.
For packets outbound from the private network, NAPT would translate
the source IP address, source transport identifier and related fields
such as IP, TCP, UDP and ICMP header checksums. Transport identifier
can be one of TCP/UDP port or ICMP query ID. For inbound packets, the
destination IP address, destination transport identifier and the IP
and transport header checksums are translated.
A NAPT router in figure 2 may be configured to translate sessions
originated from N-Pri into a single external address, say Addr-i.
Very often, the external interface address Addr-Nx of NAPT router is
used as the address to map N-Pri to.
There is a large pool of resources describing NAT (Network Address Translation), which is available if you search "nat explained". A great resource is What is NAT and how it work tutorial.
The most important detail is that commonly we use NAPT (commonly used as PAT - Port Address Translation) (Network Address and Port Translation), alongside NAT.
When a device needs to use the Internet, it must open a local (source port) and send the IP request to the other end. For example, a notepad with source IP address 192.168.1.2 needs to communicate with a web server at 216.58.212.35.
It fires up random source port 1234 and requests information from target port 80 (HTTP - Web page).
This goes through the networks NAT device, which stores the information 192.168.1.2/1234 with the next information that it computes, and sends the request as 46.103.93.105 (its own IP public IP) and a new source port, for example 2345.
The web server responds to the NAT device, which in turn finds the correlated information (source port 2345 targets 192.168.1.2/1234). The notepad receives the information and displays it to the user.
The router adds information to the request header sent to the server that allows it to look up the sender when the reply is received. This is usually accomplished by using a table stored in the router's memory that maps the PC's address to the token added to the header.
I am new to IPs and subnets. I can calculate it and know the basics. Unfortunately, I couldn't find an answer to this question.
IPs ending on 0 designate the network address. Commonly, this address defines the route which traffic will be sent to.
Imagine that you have two routers connected by serial, so you have two subnets with 2 different private IPs: 10.0.0.0/8 and 11.0.0.0/8. /8 defines the address length in bits (Class A IP). First router will route traffic to 10.0.0.0 and second router will route traffic to 11.0.0.0.
IPs ending on 255 designate the broadcast address, a special address used to deliver packets to all hosts in the network.
For example, in case of ARP is very useful to locate/query other device's MAC by sending to the broadcast address a packet where you ask who has the MAC of a specefied IP. All the devices will capture your ARP request and if any of them has the specified IP, then you'll get a response with the MAC, but notice that you can be tricked since MAC can be impersonated (ARP Spoofing).
Regards.
I recently found that packets are encapsulated within ethernet frames. Packets use IP addresses, frames use MAC addresses.
Why aren't IP addresses used in ethernet frames for routing? I understand that when trying to access a basic website, the computer goes to a DNS to find the IP address relevant to the user-entered domain name. How do computers find the correct MAC address?
Really, how are MAC addresses used in routing internet traffic?
Thanks
IP packets aren't always encapsulated in Ethernet frames. There are other physical media such as ISDN, etc. When packets are routed, IP addresses are used to determine the next hop and the physical address is used to physically identify the interface serving as the next hop. Only the former (determining next-hop) is usually called routing.
To answer your second part, MAC addresses are discovered through ARP (Address Resolution Protocol) in IPv4 & ND6 (Neighbor Discovery) in IPv6.
Update:
The destination IP address in the IP header is the final destination. In the process of routing (at each hop), you get the next hop's IP address to (eventually) reach the final destination from the routing table (this could be a default gateway's IP address). To send the packet to the next hop, you need its MAC address. While hopping through intermediate links, the IP address in the IP header don't change - only the MAC addresses change.
Bit late but still here is my answer :) ...
To send data you need two address, the MAC address and the IP address.
Basically the sending host will ARP for a MAC address, this occurs when the local host doesn't know the MAC address of the host it has an IP address for or it will ARP for the default gateway MAC address (if it doesn't already know it) if the IP address in on a different subnet/ network. Once it obtains a MAC address the IP packet is encapsulated in a L2 frame and sent across the media. If the IP packet is meant for a host on a different subnet/ network, it will be sent to the default gateway, this router will de-encapsulate the L2 frame (remove and discard it) check the IP address and will forward it. For the router to do this it needs a MAC address to send it over the media, It will look up the next hop in it's routing table, encapsulate the IP packet with the same source and destination IP address that was sent from the original host into a new L2 frame. This time the MAC address for the source address will be that of the forwarding interface of the router, and the receiving interface of the next hop will be the destination MAC address. This will continue from hop to hop until it reaches the final host, each time the MAC addresses will change, but the original IP address will remain the same.
Here's the key point -- there can be more types of packets than INTERNET traffic. You could be using IPX, which is non-routable. How do clients identify each other? By the MAC address.
Routing != Addressing, which is really where the MAC comes into play.
In order to be routed, the OSI model adds a layer to allow for path discovery to the next gateway. This layer is responsible for routing, but knows nothing about the MAC address.
As a side note, at the hardware level, MAC addresses ARE used by switches, but not for routing. From How Stuff Works:
The switch gets the first packet of data from Node A. It reads the MAC
address and saves it to the lookup table for Segment A. The switch now
knows where to find Node A anytime a packet is addressed to it. This
process is called learning.
In this way, a switch can make sure that traffic is only outputted to the correct port. This isn't accomplishing routing so much as reducing network congestion. Only broadcasts and traffic destined specifically for that MAC address should be sent out the port.
Recently I have been thinking about the same and came upon this question. Here is my answer to this question. Actually MAC address is needed for correctly sending the packet to right destination. This is specially true when packet is needed to sent over a VLAN. There can be multiple switches/routes connected on that VLAN over multiple physical interfaces. However IP Routing is unaware of these physical interface. It only knows about the logical connectivity. For example, route 10.10.10.0/24 is reachable via VE/VIF0.10(logical VLAN interface) and/or nexthop neighbor is 20.20.20.1. There could be multiple interfaces under VLAN 10. Then to which interface packet is sent out? This is where ARP comes in the picture. ARP helps to discover the MAC address associated with the next-hop IP address. When switch/router learns the nexthop MAC. along with that it learns the physical interface also via which that MAC is reachable. Hence while routing packet, firstly MAC corresponding to the destination IP is searched and then the physical interface associated with that MAC is searched. Finally packet is sent out via that physical interface. The MAC corresponding to that destination IP is used as destination MAC. In absence of this, routed packets will always be flooded in the outgoing VLAN.
Hope this helps.
Thanks.
Answer: MAC addresses are not used in the process of routing of a packet.
segment -> transport layer (TCP ports)
packets -> network layer (IP addresses)
frame -> data link layer (MAC addresses)
bits -> physical layer (electric/optical signals)
Create your own packet/segment visit http://wirefloss.com/wireit/
There are 2 models (TCP/IP and ISO/OSI)
In detail:
Your app has some data. This is encapsulated by mentioned layers. Encapsulation means that a header with fields is added at each layer. If your data never leave the local network the MAC address will be the same. Once your data needs to be delivered outside your network the frame header is stripped by router and is replaced by router fields.
UPDATE 2021: Some people seems never heard of ISO OSI model and put this answer as incorrect.
I am just preparing for a test in college about networking.
I'm currently trying around with sub netting and I found out that two devices attached to a layer 2 switch can talk to each other although they have different subnets!
Device A: 192.168.0.1 subnet mask : 255.255.255.0
Device B: 192.168.1.1 subnet mask : 255.255.255.0
The question is why I can ping from device A to B and vice versa?
There's no router, just the two devices and a switch.
From my understanding they should not see each other.
The OS should not even send the ARP request when the unknown IP is in a different subnet.
Could this be a caching issue?
Many Cisco Layer 2 switches are capable to ping the connected systems.
It is possible that your computer might have a route entry that sends a packet which matches no other specific route entry to your router. This is also called as the default gateway. Conventionally the computers in the same subnet are connected directly and most of the times, do not go through the gateway.
To explain it more clearly, If you're on a Linux machine, run route -n.
Destination Gateway Genmask
14.0.1.0 0.0.0.0 255.255.255.0
0.0.0.0 172.16.80.1 0.0.0.0
The first entry has a destination ranging from 14.0.1.0 to 14.0.1.255. The gateway for this match is 0.0.0.0. The table implies that these systems are connected directly. On the other hand, the entry 0.0.0.0 in the destination field will get matched when the packet matches none of the other entries. The gateway for this is the router address (which in my case in 172.16.80.1). All the packets that do not have IPs in the range that I specified above go to the router for further routing. Once the router gets the packet, it takes the further decision based on its routing information that it posses.
In your case the router happens to know that the other subnet is attached to it and hence passed your packet onto that subnet.
Have a closer look at the ARP. The ARP would be addressed to the router in your case of pinging to the other subnet. On the other hand, if pinging within the network, the ARP would be to the destination directly. This is the conventional case. Of course, it all depends on the routing tables in your system. You can always make all packets go through the gateway or no packets to go through the gateway.
Can a computer with an IP address of class C like 192.168.0.1 and subnet mask 255.255.255.0 communicate and share resources with another computer having the IP 192.168.1.1 and the same subnet mask 255.255.255.0 ? I'm asking this because the first 3 octets in this case tell us that these IPs are running on different networks (network 192.168.0.0 and network 192.168.1.0).
Also, does "network" mean the same thing as "subnetwork" (or "subnet") in this context?
Thank you!
To fully understand how computer networks work, you need to take a look at OSI model (or in practice - TCP/IP or DoD model. For your question you need to look at first three layers: physical, data link and network.
Physical connection is self explanatory, and represents direct connection via some medium (copper, glass, air).
When a host A tries to send a packet to host B first thing it will do is look at the destination IP address and based on it's own IP configuration determine if host B is in the same subnet as a host A. This is done as Eugen Rieck explained to you already: subnet mask bits are used to mask the bits of the IP address (logical AND operation). Now, we have two cases:
Host A and B are in the same subnet.
Host A and B are not in the same subnet.
You should note that on layer 2, which network adapters use to send and receive frames, there is no IP addresses (which are present on layer 3) but instead the communication between devices is done by using MAC addresses. Because of that, host can directly communicate only with hosts in their subnet (1st scenario). For sending a frame host A needs the MAC address of host B. So host A first looks up the MAC address mapped to the IP address of host B in his ARP table. If he can't find it, it sends broadcast ARP request asking all host on the subnet who has that specific IP. If he gets a response it adds the MAC address of the host he got the reply from and builds a packet with destination MAC address of that host and IP address of that host.
If both hosts are not in the same subnet (2nd scenario) the packet is sent to default gateway which is responsible for finding a route to the destination. The crucial point to make here is that even if the destination MAC address in this case is the MAC address of the router (default gateway), the destination IP address is still the IP address of host B as in the first scenario. As the packet flows from router to router the source and destination MAC addresses will change, as they are locally significant, but the source and destination IP address will stay the same. This is how every layer provides a service (so to speak) to upper layers, and upper layers use it transparently without needing to know what is happening below.
So you have:
1st scenario.
----------------
L2:
Src MAC: host A
Dst MAC: host B
----------------
L3:
Src IP: host A
Dst IP: host B
----------------
2nd scenario:
----------------
L2:
Src MAC: host A
Dst MAC: router
----------------
L3:
Src IP: host A
Dst IP: host B
----------------
To sum it up (the answer #Eugen Rieck already gave you):
Two hosts which are not in the same subnet as in your example (192.168.0.1/24 and 192.168.1.1/24) will not be able to communicate on layer 2, and will require a L3 capable device such as router to act as a default gateway and to route the traffic between two networks (broadcast domains) for layer 3 connectivity.
Yes and no:
Yes: Those two computers can communicate, if there is a (properly set up) router in between and both sides have knowledge of it.
No: Those two computers can not communicate, if simply wired to the same dumb switch.
Rule of thumb: IP & SNM must be identical for all participants to allow direct communication.
You'd need to change the subnet mask to 255.255.254.0, or use a router or layer-3 switch to communicate.
Network generally means the whole network you're referring to, while subnet refers to a specific separate portion of it. However, the terminology is pretty loose.