The internet can be accessed from nearly everywhere and on nearly every device but if you have paid for data, does it matter which device you use?
Most of the big mobile operators believe it does. Many incorporate higher charges for tethering - using your phone to connect tablets or laptops to the internet.
Unlimited data plans once were common but now are rare.
Mobile operators began changing their policies when more and more people started using smartphones. Now most data plans are capped, in the UK for example to typically between 500MB and 1GB.
But operators have been criticised for charging twice for the same data. It is argued that if the data has already been paid for, it shouldn't matter on what device you use it.
"To me, this is a bit like a gas company selling you gas to heat your home with and then you say 'could I use it for the oven to make my dinner?' and they say 'hold on, that'll be a bit extra'," says Martyn Hocking, editor of UK consumer magazine Which?
"To us, once you've bought the data, what you do with it and how much of it you use should be entirely up to you."
Mobile data usage has increased quite dramatically over recent years
"It feels as though they've maybe given us more data allowances than they really thought we'd use and there's a little bit of panic in the industry," he added.
Most new smartphones using either Google's Android, Apple's iOS or RIM's BlackBerry operating systems can tether. Microsoft's forthcoming updated Windows 7 Mango will also allow this feature.
But networks are less happy about letting users loose with different devices without purchasing new allowances from them first.
Experts say there are a number of reasons for this. One is that mobile companies are obviously keen to maximise revenue from selling data and the dongle could become redundant if tethering takes off.
"Those who do learn to tether won't be buying those dongles for PCs so that's a business model on its way out of the door," says Flora Graham, of the New Scientist.
No matter how you justify it, if you enable unpaid tethering on a network that doesn't allow it you are a thief
James Kendrick, ZDNet
Already around 10% of mobile network 3's traffic is from tethering.
Another reason could be that the networks aren't prepared for everybody using their full data allowance which they have already paid for.
"A lot of the networks depend on the fact that if you have a lower cost plan then you won't use much of that data," says Ms Graham.
"Most people do not use the calls, text and data in their contract. The vast majority of people could take a look at their plan and go for a less expensive one."
When installing the data cap, O2 justified the decision by saying that only around 3% of people - the so called super-heavy users - use over their limits.
What many users don't realise is that web browsing is more data-intensive on larger devices.
Using the web on a tablet often uses twice the data of a mobile phone for viewing the same information.
The two biggest networks in the US have clamped down on what they say is "illegal" tethering.
Instead of allowing users to install unofficial apps to workaround any barriers, operators are now re-directing them to a page indicating the cost of upgrading to a tethering plan.
Windows Phone 7 is still yet to offer tethering using its operating system
AT&T says its aim is "fairness for all of our customers" by making all users pay the additional fee. But even analysts are divided as to whether it can legally justify charging more.
"The simple truth is that those who go the unofficial tethering route are stealing service from the carrier, with the exception of those lucky customers whose plans allow tethering as part of the basic service,"
writes James Kendrick of ZDNet.
"No matter how you justify it to yourself, if you enable unpaid tethering on a network that doesn't allow it you are a thief."
Mr Kendrick believes that everyone should play by the rules of their contract whether they like it or not because it is the operators network, not the customers.
But others, even at ZDNet argue that this is in neither the users' or the operators' best interests.
"I think carriers are taking advantage of [consumers]," says Steven J Vaughan-Nichols, also of ZDNet.
"It's worse still from a business perspective. In the long run, since fewer and fewer operators are offering unlimited bandwidth, if someone chooses to share it, they will be paying more for it. Why would an operator discourage that?
"They are annoying their customers and cutting down on a future revenue stream."
Data usage is generally higher on a tablet than it is on a phone
In Europe though, the tide is changing with a number of operators no longer separating mobile and tethered data.
O2 and 3 have offered tethering without extra charge since earlier this year and Vodafone has just begun including it on new and updated contracts.
Orange only lets iPhone customers do it and both Orange and T-Mobile charge more for packages that allows a personal hotspot to be switched on.
But there are concerns that the information is not being disseminated to mobile users. Vodafone's website, for example,
still states that one has to buy
a special tethering "bundle" to use the service with the iPhone.
This is no longer the case.
And Which? found that many other operators have few details about tethering and these cannot easily be found by using their website alone.
While operators didn't invent tethering, most have decided its benefits are something that they should charge for, even if their customers don't use anymore data than they've already bought.
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