What the APN settings mean
Android's APN settings contain vital information required to use internet data while you are on a mobile network (that is, not connected to wifi).
In most cases, you should not have to change these as your phone will automatically choose settings appropriate for your carrier. In some cases, you will need to set these up following directions by your carrier.
This is simply a name to give this APN setting in the menus. This has no effect whatsoever on your data connection, and you can set it to what you like. Use it to distinguish between multiple APN settings that are configured.
The most vital and important setting in the group, APN stands for Access Point Name. Whenever your phone needs to open a data connection with your carrier, it quotes an Access Point Name. This is then used by the carrier to determine various settings on their end.
The APN you configure must match one of the APNs your carrier accepts, and this varies by carrier, so you should ask them (or search for their recommended APN settings on their website) to find out the correct name to put in this field.
If the APN does not match one that your carrier supports, or if it's left blank, this APN setting won't work at all.
An optional field allowing you to specify the address of a HTTP proxy to use for all web traffic over this connection. Carriers sometimes use HTTP proxies to modify websites for your device, or to improve the speed of commonly access web pages and resources by caching them.
It's relatively rare for a carrier to require that you use an HTTP proxy for your connection. In addition to this, many carriers use HTTP proxies that don't even require this field to be set, known as transparent proxies.
As always, consult your carrier's recommended APN settings to see whether they recommend that you add the address of their proxy server into this field. Many carriers won't use or require a proxy, meaning this setting should remain empty.
The address given may be an IP address (for example, four numbers with dots such as 10.0.0.1) or the hostname of the proxy server.
This field relates to the above (Proxy) field, and if a proxy is being used, both must be set. To use an HTTP proxy you must not only supply the address of the proxy but also the port number to use for connecting to it.
Consult your carrier's recommended APN settings to see what port number you need to use if you are using an HTTP proxy. If you do not need to use an HTTP proxy this should remain empty.
Common port numbers for HTTP proxies may include 80, 3128, 8080, or any high number ending with the digits "80".
Username / Password
In rare cases a mobile carrier may require a username and password in order to access data. If this is the case, enter that username and password in these fields.
In most cases this will be left blank.
This username and password is used to authenticate the PPP (Point-to-Point Protocol) connection from your phone to your carrier's servers. It is rare that a carrier would require this, as they can already identify you in a more secure way from the information in your SIM card, and using a username and password does not provide significant additional security. In cases where a carrier does require these fields, they will usually just require a rather general word in both, rather than anything unique.
In general, you shouldn't fill these with the username and password you use to access your account on the carrier's website. It is usually unrelated.
Note: Despite what their positioning might suggest, these fields do not relate to the Proxy fields above them, and do not represent the username and password given to the proxy server. Unfortunately, Android does not support HTTP proxy servers that require authentication.
In modern smartphones this setting is rarely used.
This should usually be left blank.
This setting likely refers to the address of a WAP Gateway Server, which is a feature that was used in mobile devices which were incapable of accessing normal websites.
In order to send and receive MMS messages, you phone needs to connect to an MMS Gateway Server as specified in this field.
This field needs to be set to the MMSC address (which looks like a web address) provided to you by your carrier. If this is not set, or set incorrectly, you won't be able to send or receive MMS messages. People who send MMS messages to you may not be notified that you don't receive them.
MMS messages work similarly to SMS messages, except that instead of including just 160 characters of text, they may include photos and short videos, along with text that is longer than 160 characters. Your phone may not make it clear when you are sending an MMS message, as its built-in messaging application may automatically choose whether to send a message as SMS or MMS based on whether the message contains images, or even its length.
MMS messages require a data connection, but many carriers will not charge money for the data transfer that they use, instead charging a certain fee per message sent (which will usually be much higher, anyway). Thus, most carriers won't consider the data used by sending or receiving MMS messages towards your internet data limits.
Note: Some carriers require the use of a different Access Point Name (APN field, above) for MMS and for internet data use. When this is the case, you will need to have multiple separate APN settings in your device with different Access Point Names configured, and only the MMS-specific one will require the MMSC field to be set.
After changing the MMSC, MMS proxy, or MMS port fields, or any other fields in the APN you use for MMS messages, you may want to test that MMS messages are still being sent or received correctly - for example, by sending a message with a picture to yourself. This may, however, cost money.
MMS proxy / MMS port
These settings refer to an HTTP proxy that will be used only for communicating with the MMS Gateway Server. As with the MMSC setting above, these will need to be set according to the settings provided to you by your carrier. If they are not set correctly, you may still be unable to send or receive MMS messages, and people who send you MMS messages may not be notified that you don't receive them.
This specifies the protocol that your device should use when communicating with the MMS Gateway Server. In modern devices, this usually defaults to WAP 2.0. If your carrier requires the use of the older WAP 1.2 protocol, or another protocol, they should include this information in the APN settings they provide to you or document on their website.
MCC / MNC
The Mobile Country Code (MCC) and Mobile Network Code (MNC) fields together specify which carrier you are using. Each carrier has their own unique MCC and MNC values.
These will usually be set according to the SIM card that is inserted in your phone and usually won't need changing. Your device may even prevent you from making changes to these fields, rendering them display-only fields.
Android's APN settings are all carrier-specific: when you create an APN setting, it'll be used only when you are using the carrier network specified in its MCC and MNC fields. If you take out the SIM card and replace it with one for another carrier, the APN settings you previously created will probably no longer be visible. If so, this is not because they have been deleted, but because Android sees no need in showing you APN settings for networks that don't match your SIM card. In reality, Android devices usually ship with a large number of pre-configured APN settings, but only the ones that match the MCC and MNC corresponding to the provider of your SIM card will be listed.
Changing the MCC and MNC fields for an APN setting is not recommended (if your device allows it). Instead, if you want to configure an APN for a different carrier, it's recommended to insert a SIM card that corresponds to that carrier first.
This specifies the method that your device may use to supply your username and password to the server for your PPP (Point-to-Point Protocol) connection. Available choices include PAP, CHAP (or Microsoft CHAP), either, or neither.
If you carrier does not require you to supply a username and password in your APN settings, this setting probably has no effect. For unknown reasons, many carriers tend to specify what you should put here anyway, even if they don't require a username or password.
Note that neither PAP nor CHAP are particularly secure protocols, and that carriers won't be relying on them for actual security purposes.
This is the second-most important of all fields, along with the "APN" field itself. It specifies what types of data connection should use this set of APN settings.
In many cases, your carrier will require a different set of APN settings for MMS and for Internet data, for example. In these cases, it is this field that determines which set of APN settings is used for which.
The APN setting that is to be used for MMS should have "mms" in this field. The APN setting that is to be used for internet data should have "default" in this field (though there are other values that can have equivalent meanings).
Other APN types that may sometimes be used are "dun" for dial-up networking (this is not necessarily the way that modern smartphones do tethering anymore) or "supl" for a feature called "Secure User Plane Location", a component of Assisted GPS (AGPS).
It is possible for this field to include multiple values, separated by commas (without any spaces).
In cases where you only have a single APN setting that supports any type of data connection, this field may contain the special value of "*" (a single asterisk). This indicates to your device that it may use this APN setting for any data connection.
As usual, you should follow your carrier's recommendations for what to include in this field for each APN setting.
If you have multiple sets of APN settings that support Internet data (that is, they have APN type values like "default", or "*") this allows you to choose which one of them is currently enabled.
If you only have one set of APN settings that support Internet data, or if this isn't one of them, this field will probably be disabled and would not make sense anyway.
Checking this box is equivalent to selecting an APN setting from the menu in the "APNs" screen.
Information about this setting is hard to come by, but it appears to relate to devices and carriers that are capable of switching between CDMA technologies (including EvDO) and LTE.
This should be set to the setting that is supplied by your carrier or listed under the recommended APN settings on their website. If it is set incorrectly, data may not work.