There has been a lot of sites affected over the past few months by bots accessing sites, not sure if it is specific to phpBB. In that case I don't believe it's a DDOS attack but scraper bots harvesting content. It amounts to DDOS attack because they are aggressive and overwhelming the sites resources.
If someone is specifically attacking your site with DDOS it's usually multiple IP's, hundreds or even thousands of them. It's very easy to stop DDOS from one IP, not so easy from multiple IP's unless you are prepared for it. Most DDOS attacks are carried out by guns for hire. they rent their bot network for $X per hour, $X depends on the scale of the attack.
A single IP attempting various exploits is very common. I see hundreds of occurrences each day for this. I checked my logs for this IP and it doesn't appear, I did however have a German IP around the same time this was posted with short burst of 36K requests attempting an SQL injection attack which is very out of the ordinary because of the amount of requests.
“Results! Why, man, I have gotten a lot of results! I have found several thousand things that won’t work.”
Attributed - Thomas Edison