Janet is the network responsible for running the country's .ac.uk and .gov.uk domains, so the attack has resulted in many universities, colleges and some councils seeing their websites go down or slow significantly.
The website for the Janet network itself is also completely down.
The attack is now in its second day, and many ICT support departments at UK universities are alerting users to service disruptions.
At the time of writing, Janet's status page indicates that computer services at a large number of academic institutions and councils around the country are currently down or significantly degraded.
#TT169600 - Still ongoing. It is affecting most services. Due to the nature of the outage we cannot give a ETA on restoration.— Jisc Major Incidents (@jiscmi) December 8, 2015
Many people at these institutions have taken to Twitter, complaining that their internet connection or access to certain services has been disrupted, leaving some students unable to meet deadlines and do their work
At the University of Liverpool, the computing services department said: "The University is currently experiencing intermittent interruptions in network connectivity to external websites/email."
"This is being caused by Janet, the high-speed network for the UK research and education community, which is also affecting other institutions."
Apparently there has been a cyber attack on UK university internet networks. At least that's what my lecturer said.— Hammy (@Hammy_UK) December 8, 2015
Uni Internet down and having no idea what to do with my life without access to Netflix— Tipping (@SamTipping) December 8, 2015
Got up to go to library for 9am, uni internet down all morning, now 1pm n work progress = 0 #storyofmylyf— Emily Woodworth (@emilykwoodworth) December 8, 2015
when the uni internet goes down and you can't submit your deadline....— Helen Prince (@ImHelenPrince) December 8, 2015
"Janet engineers are working to restore, and stabilise, their service as quickly as possible."
The Janet network is managed by Jisc, a publicly-funded body, which has been providing updates about the attack on its Twitter account.
As of 8 December there were some improvements in certain regions, but most services continued to be affected. Jisc were unable to provide an estimated time when the service could be restored due to the nature of the attack.
A DDoS attack is when an online service is brought down or interrupted by being overwhelmed by traffic from multiple sources.
Hackers can perform a DDoS attack by creating 'bot' computers by infecting thousands of home computers with malware, and instructing them to all attack a certain site at once, bringing it down.
DDoS attacks typically last for around 24 hours, but the current attack on Janet has now been going on for much longer than that.
It was this kind of attack that was used on TalkTalk in October, bringing the service down and allowing hackers to steal the information of tens of thousands of customers.