As an aside on meanings, I'd avoid notion here, because it means a vague sense, or an awareness of something exists or is a possibility, without a deep understanding of it. To have a notion of IT infrastructure, would mean that you have some vague idea that IT systems sometimes need some sort of infrastructure, but no more than that. Maybe that's exactly what you want, or maybe notion is favoured in your field for some other reason, but generally I'd go for the stronger understanding.
To the question of grammar, while the two work slightly different, they end up meaning the same thing. Using understanding as a suggestion instead of notion, then:
understanding of IT infrastructure
IT infrastructure understanding
Are all acceptable. While one uses a preposition and the other turns "IT infrastructure" to an adjectival use, both are clear in meaning the same thing.
The only difference in the last two is of course the hyphen. This is allowed, but not required. I'd choose on them by trying to judge how likely someone whose eye was caught by "infrastructure understanding" to not realise that the earlier "IT" belonged with the phrase, and hyphenate if I thought it likely. Here, I would judge it unlikely and not hyphenate, but there's no strict rule on this decision (there is in some style guides).
Having decided the non-hyphenated form is the better of the last two (or perhaps disagreeing with me, and favouring the hyphenated), in deciding between that and the form using of, I personally would consider the of form slightly clearer and stronger, and hence use it if the phrase came up once or twice in a piece. On the other hand, the slightly greater concision cohesion as a unit means I would favour it if I was going to end up using it a great many times.
I'd also decide based on things like avoiding repetition of of close together, though I'd probably avoid it here by opting for "Developing an understanding of IT infrastructure" which loses the first of instead. (Not that repeated of must be avoided, it can just scan better if you can do so without twisting into something barbarous).
In all though, while there is strictly a grammatical difference between the two, that difference is in the means, not in the end. They're both acceptable, and both convey the same meaning.