Here are all the interchanges on the beltway. As for the old exit numbers, the Maryland side was renumbered in 11-1979 (Prince Georges County, up to current exit 27) and 5-1980 (Montgomery County, north of current exit 27). The Virginia side was renumbered in 11-2000. The reason for the numbers in the 170s is that Interstate 95 uses the eastern half of the Beltway.
new old intersecting route
------- enters Maryland as I-95 north/I-495 north
2 38 Interstate 295
3 37 210
4 37A 414
7 36 5
9 35 337
11 34 4
15 33 214
16 -- USAir Arena
17 32 202
19 31 US Route 50
20 30 450
22 29 Baltimore-Washington Parkway
23 28 201
24 -- Park & Ride
25 27 US 1
-- 26 Interstate 95
------- becomes I-495 west
27 26 Interstate 95
28 25 650
29 24 193
30 23 US 29
31 21 97
33 20 185
34 19 355
35 19 Interstate 270
36 18 187
------- becomes I-495 south
38 17 Interstate 270
39 16 190
40 16 Clara Barton Parkway
41 15 Clara Barton Parkway
43 14 George Washington Memorial Parkway
44 13 193
45 12 267 (Dulles Airport Access Road)
46 11 123
47 10 7
49 9 Interstate 66
50 8 US 50
51 7 650
52 6 236
54 5 620
57 4 Interstate 95/Interstate 395
------- becomes I-95 north/I-495 east
170 4 Interstate 95/Interstate 395
173 3 613
174 3A Eisenhower Ave
176 2 241
177 1 US 1
sources:
http://web.mit.edu/spui/www/boston/otherexitnumbers.html#i495va
http://web.mit.edu/spui/www/freeway/mdi495.html