The Royals led 3-0 after an hour of this cup tie, which proved to be a comfortable evening on their travels.
They had an early chance after just two minutes when Conor Bailey and George Barker combined down the left before playing the ball inside to Lee Wort on the edge of the box but his shot was charged down by the home defence.
Portland had their first sight of goal five minutes later when Archie Woolford and Luke Sheehy broke forward quickly from half way into the box and after the ball was blocked and ran to Brin Doyle his shot was hacked away.
The visitors looked more of a threat when they went forwards and on eight minutes a break down the left allowed Harrison Brook to cross the ball to Wort on the edge of the six yard box and he tried to guide his shot home but keeper Harrison Bruton got down well to prevent it going in.
Two minutes later a quick throw in from the right found Wort, who saw his shot saved by Bruton. The rebound went to Brook and his first time shot rolled narrowly wide of the post.
At the other end Doyle and Sheehy exchanged passes in midfield before Doyle took the ball up to the penalty area but with only keeper Steven Mowthorpe to beat he blasted the ball over the top.
Joe Wickham then cut in from the left edge of the box and his low shot had to be saved at his near post by Mowthorpe.
Portchester got their noses in front on 23 minutes after Wort played Barker in down the right and his shot from 12 yards out was blocked. The visitors picked up the ball again and it was worked out to the right to Charlie Williamson, who beat his marker before cutting inside and from the byline he hit a low cross into the six yard box which a defender could only turn into his own net.
Portland had the first chance of the second half when a cross from the right found Doyle on the penalty spot but his shot was smothered by Mowthorpe and Sheehy was unable to prod home the rebound.
At the other end Williamson’s cross to the far post was headed clear as far as Marley Ridge on the edge of the box and he let fly with a shot which Bruton had to tip over the bar. The Royals doubled their lead from the resulting corner on 52 minutes, which was cleared to Jake Raine outside the penalty area on the left and his cross back into the box was missed by everybody and it flew straight into the net.
The visitors wrapped up this game following a quick break forwards after the defence broke up a Portland attack on the hour mark. Brook fed the ball through to Wort, who took it on into the penalty area before hitting a low shot past Bruton and inside the far post.
The game then drifted to its conclusion as spells of heavy rain came and went. On 63 minutes Williamson’s cross from deep on the right flew across the six yard box and Brook was only inches away from applying the finishing touch.
Wort and Ridge then swapped passes on the edge of the box before Ridge unleashed a shot which Bruton was relieved to watch fly narrowly wide of the post.
Wort then tried his luck from just inside his own half and Bruton was able to back peddle quickly enough to pluck the goal-bound shot out of the night sky and at the other end Sheehy’s shot from 25 yards out hit the top of the crossbar.
Barker beat the home keeper with his shot from the edge of the box on 75 minutes but a defender managed to divert it wide for a corner.
With five minutes remaining a pass from the left found Jacob Coombs just outside the six yard box and Mowthorpe had to get down quickly to smother his shot.
In injury time Bailey’s fierce free kick from 35 yards out went straight into Bruton’s gloves and at the other end Tom Butcher set up a chance for Sheehy but the ball bounced away from him at the crucial moment.