In many cases, it provides additional services that are useful for a home LAN, including three that are the subject of this experiment:. This eliminates the need for a user or network administrator to configure IP address settings manually on every host. It also allows a "pool" of IP addresses to be shared among many hosts; if a host leaves the network, the IP address it was using can be reassigned to someone else just entering the network.
After these four stages, the client is configured with an IP address. For web browsing and similar use cases, hosts on the Internet are referred to by human-readable names, rather than by their IP addresses. The Domain Name System DNS is a system that stores records mapping those names to the corresponding IP address, and allows those records to be queried name resolution.
Many gateways also act as a DNS server, i. Each server in the chain refers server that initiates the query to the next server in the chain, until it reaches one that can fully resolve the requested name.
This answer explains why. Suppose the DNS server does not know the IP address associated with this name, and also does not know the address of a DNS server that does know the address associated with this name. The following figure describes the iterative procedure it will follow:.
Thus, any DNS server that knows the address of some root servers can eventually find out the IP address associated with any name. The hierarchy of the name servers in this example and examples of other name servers at similar levels is shown in the following figure:. This is a method of method of remapping one IP address space into another by modifying source and destination IP addresses in packet headers as they pass through the gateway.
Publicly routable IPv4 addresses, which are scarce, are not generally assigned to every host in a home network. Instead, the Internet Service Provider will provide one IP address, which is assigned to the Internet-facing interface of the gateway.
However, those addresses are not routable on the Internet. Thus, a common NAT use case is to map between private addresses used on a home LAN and the public address associated with the router, so that traffic can be routed between the LAN and the Internet.
The following figure shows how a NAT gateway in this scenario will rewrite the headers of packets that traverse it:. An entry is added to the translation table only when a packet associated with a new connection arrives from the LAN.
A workaround for this problem, port forwarding , enables a home user to proactively map a port to a particular host on the home network. For example, consider the home gateway configuration in the following image. Suppose the gateway has Hosts on the WAN can initiate a connection to We see an iterative name resolution with DNS, for the address "website.
First, the DNS server returns a list of root name servers:. Then it queries one of the root name servers "c. Next, the "c. And then "dns2. Finally, from "ns. Wait for your nodes to boot up they will turn green in the canvas display on your slice page in the GENI portal when they are ready and then log in to each node. Wait for all of your nodes to become ready to log in they will turn green on the canvas.
However, the clients in our experiment are already set up to use a university gateway for those services. Otherwise, they would not have a functional network connection and we would not be able to get in to them over SSH! To run this experiment, we will tell the clients not to use the university gateway for anything except our SSH session. However, once you do this, you should plan to finish the rest of the experiment involving these resources in the same SSH session.
Otherwise, if you stop and then resume the experiment from a new SSH session in a new location, you won't be able to access the clients anymore. When you're ready, on each of the two "client" nodes and only on the client nodes , run.
Note : as mentioned above, if your own IP address changes, you may lose connectivity to your client nodes. Here's how to access your client nodes if that happens!
On the "gateway" node, run:. The contents of the config file are:. Now that both the gateway and the clients are set up, we will observe what happens when clients use some typical residential gateway services.
The -d argument is to specify debug mode; it keeps the dhclient process in the foreground, and lets us see its output. The client does not have an IP address yet, so this message has "0.
Next, a DHCP server that receives this request in this case: our gateway at In the example below, it offers the IP address " The server also includes other DHCP options in the message -in this case, it informs the client of the default gateway to use option 3 and the name server to use option 6.
The client responds with a request for the IP address that was just offered. The other DHCP servers will notice the request since it is sent to the broadcast address, they will all receive it and understand that the client does not accept their offer:.
You can also see that the client has been configured to use the gateway as the name server, as specified in the DHCP options. DHCP is completely unrelated to this. It is a way for devices to shout to the local network asking for an IP address and other related information like the gateway address, name servers etc to be given to them.
Again, a home router usually has a server to serve the clients in the local network. Maybe this causes the confusion to you. You can set fixed IPs, or you can use NAT to divert traffic through a firewall to some other machine etc.
DHCP is a protocol for configuring hosts, and runs periodically to keep their addresses and other configuration up to date. NAT a process is for rewriting packets as they are forwarded across a router.
To be more specific it does translate the addresses and sometimes the ports. As I understand it, in NAT, the router which connects the private network to the public internet has one IP address, which all traffic for the private network is directed to; the router then uses port numbers to figure out which device on the private network the traffic is for.
In contrast, with DHCP, each device on the network is assigned it's own public IP address which traffic for it is directed to.
My confusion stems from hearing that both can be used simultaneously, which seems to contradict the idea that with NAT, there is only one IP address for the entire private network whereas with DHCP, there is an IP address for each device on the network. If they are mutually exclusive, where is each typically used? Edit: The answer below explains it really well — I was being dumb and not realising that DHCP can assign any "type" of IP address, it's just a protocol for devices to join a network?
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